The Student's Elements of Geology 4

The Student's Elements of Geology 4

As all the crystalline rocks may, in some respects, be viewed as belonging to

one great family, whether they be stratified or unstratified, metamorphic or

Plutonic, it will often be convenient to speak of them by one common name. It

being now ascertained, as above stated, that they are of very different ages,

sometimes newer than the strata called secondary, the terms primitive and

primary which were formerly used for the whole must be abandoned, as they would

imply a manifest contradiction. It is indispensable, therefore, to find a new

name, one which must not be of chronological import, and must express, on the

one hand, some peculiarity equally attributable to granite and gneiss (to the

Plutonic as well as the ALTERED rocks), and, on the other, must have reference

to characters in which those rocks differ, both from the volcanic and from the

UNALTERED sedimentary strata. I proposed in the Principles of Geology (first

edition volume 3) the term "hypogene" for this purpose, derived from upo, under,

and ginomai, to be, or to be born; a word implying the theory that granite,

gneiss, and the other crystalline formations are alike NETHERFORMED rocks, or

rocks which have not assumed their present form and structure at the surface.

They occupy the lowest place in the order of superposition. Even in regions such

as the Alps, where some masses of granite and gneiss can be shown to be of

comparatively modern date, belonging, for example, to the period hereafter to be

described as tertiary, they are still UNDERLYING rocks. They never repose on the

volcanic or trappean formations, nor on strata containing organic remains. They

are HYPOGENE, as "being under" all the rest.

 

From what has now been said, the reader will understand that each of the four

great classes of rocks may be studied under two distinct points of view; first,

they may be studied simply as mineral masses deriving their origin from

particular causes, and having a certain composition, form, and position in the

earth's crust, or other characters both positive and negative, such as the

presence or absence of organic remains. In the second place, the rocks of each

class may be viewed as a grand chronological series of monuments, attesting a

succession of events in the former history of the globe and its living

inhabitants.

 

I shall accordingly proceed to treat of each family of rocks; first, in

reference to those characters which are not chronological, and then in

particular relation to the several periods when they were formed.

 

 

CHAPTER II.

 

AQUEOUS ROCKS.-- THEIR COMPOSITION AND FORMS OF STRATIFICATION.

 

Mineral Composition of Strata.

Siliceous Rocks.

Argillaceous.

Calcareous.

Gypsum.

Forms of Stratification.

Original Horizontality.

Thinning out.

Diagonal Arrangement.

Ripple-mark.

 

In pursuance of the arrangement explained in the last chapter, we shall begin by

examining the aqueous or sedimentary rocks, which are for the most part

distinctly stratified, and contain fossils. We may first study them with

reference to their mineral composition, external appearance, position, mode of

origin, organic contents, and other characters which belong to them as aqueous

formations, independently of their age, and we may afterwards consider them

chronologically or with reference to the successive geological periods when they

originated.

 

I have already given an outline of the data which led to the belief that the

stratified and fossiliferous rocks were originally deposited under water; but,

before entering into a more detailed investigation, it will be desirable to say

something of the ordinary materials of which such strata are composed. These may

be said to belong principally to three divisions, the siliceous, the

argillaceous, and the calcareous, which are formed respectively of flint, clay,

and carbonate of lime. Of these, the siliceous are chiefly made up of sand or

flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter

with a certain proportion, about a fourth in weight, of aluminous earth; and,

lastly, the calcareous rocks, or limestones, of carbonic acid and lime.

 

SILICEOUS AND ARENACEOUS ROCKS.

 

To speak first of the sandy division: beds of loose sand are frequently met

with, of which the grains consist entirely of silex, which term comprehends all

purely siliceous minerals, as quartz and common flint. Quartz is silex in its

purest form. Flint usually contains some admixture of alumina and oxide of iron.

The siliceous grains in sand are usually rounded, as if by the action of running

water. Sandstone is an aggregate of such grains, which often cohere together

without any visible cement, but more commonly are bound together by a slight

quantity of siliceous or calcareous matter, or by oxide of iron or clay.

 

Pure siliceous rocks may be known by not effervescing when a drop of nitric,

sulphuric or other acid is applied to them, or by the grains not being readily

scratched or broken by ordinary pressure. In nature there is every intermediate

gradation, from perfectly loose sand to the hardest sandstone. In MICACEOUS

SANDSTONES mica is very abundant; and the thin silvery plates into which that

mineral divides are often arranged in layers parallel to the planes of

stratification, giving a slaty or laminated texture to the rock.

 

When sandstone is coarse-grained, it is usually called GRIT. If the grains are

rounded, and large enough to be called pebbles, it becomes a CONGLOMERATE or

PUDDING-STONE, which may consist of pieces of one or of many different kinds of

rock. A conglomerate, therefore, is simply gravel bound together by cement.

 

ARGILLACEOUS ROCKS.

 

Clay, strictly speaking, is a mixture of silex or flint with a large proportion,

usually about one fourth, of alumina, or argil; but in common language, any

earth which possesses sufficient ductility, when kneaded up with water, to be

fashioned like paste by the hand, or by the potter's lathe, is called a CLAY;

and such clays vary greatly in their composition, and are, in general, nothing

more than mud derived from the decomposition or wearing down of rocks. The

purest clay found in nature is porcelain clay, or kaolin, which results from the

decomposition of a rock composed of feldspar and quartz, and it is almost always

mixed with quartz. The kaolin of China consists of 71.15 parts of silex, 15.86

of alumine, 1.92 of lime, and 6.73 of water (W. Phillips Mineralogy page 33.);

but other porcelain clays differ materially, that of Cornwall being composed,

according to Boase, of nearly equal parts of silica and alumine, with 1 per cent

of magnesia. (Phil. Mag. volume 10 1837.) SHALE has also the property, like

clay, of becoming plastic in water: it is a more solid form of clay, or

argillaceous matter, condensed by pressure. It always divides into laminae more

or less regular.

 

One general character of all argillaceous rocks is to give out a peculiar,

earthy odour when breathed upon, which is a test of the presence of alumine,

although it does not belong to pure alumine, but, apparently, to the combination

of that substance with oxide of iron. (See W. Phillips Mineralogy "Alumine.")

 

CALCAREOUS ROCKS.

 

This division comprehends those rocks which, like chalk, are composed chiefly of

lime and carbonic acid. Shells and corals are also formed of the same elements,

with the addition of animal matter. To obtain pure lime it is necessary to

calcine these calcareous substances, that is to say, to expose them to heat of

sufficient intensity to drive off the carbonic acid, and other volatile matter.

White chalk is sometimes pure carbonate of lime; and this rock, although usually

in a soft and earthy state, is occasionally sufficiently solid to be used for

building, and even passes into a COMPACT stone, or a stone of which the separate

parts are so minute as not to be distinguishable from each other by the naked

eye.

 

Many limestones are made up entirely of minute fragments of shells and coral, or

of calcareous sand cemented together. These last might be called "calcareous

sandstones;" but that term is more properly applied to a rock in which the

grains are partly calcareous and partly siliceous, or to quartzose sandstones,

having a cement of carbonate of lime.

 

The variety of limestone called OOLITE is composed of numerous small egg-like

grains, resembling the roe of a fish, each of which has usually a small fragment

of sand as a nucleus, around which concentric layers of calcareous matter have

accumulated.

 

Any limestone which is sufficiently hard to take a fine polish is called MARBLE.

Many of these are fossiliferous; but statuary marble, which is also called

saccharoid limestone, as having a texture resembling that of loaf-sugar, is

devoid of fossils, and is in many cases a member of the metamorphic series.

 

SILICEOUS LIMESTONE is an intimate mixture of carbonate of lime and flint, and

is harder in proportion as the flinty matter predominates.

 

The presence of carbonate of lime in a rock may be ascertained by applying to

the surface a small drop of diluted sulphuric, nitric, or muriatic acid, or

strong vinegar; for the lime, having a greater chemical affinity for any one of

these acids than for the carbonic, unites immediately with them to form new

compounds, thereby becoming a sulphate, nitrate or muriate of lime. The carbonic

acid, when thus liberated from its union with the lime, escapes in a gaseous

form, and froths up or effervesces as it makes its way in small bubbles through

the drop of liquid. This effervescence is brisk or feeble in proportion as the

limestone is pure or impure, or, in other words, according to the quantity of

foreign matter mixed with the carbonate of lime. Without the aid of this test,

the most experienced eye can not always detect the presence of carbonate of lime

in rocks.

 

The above-mentioned three classes of rocks, the siliceous, argillaceous, and

calcareous, pass continually into each other, and rarely occur in a perfectly

separate and pure form. Thus it is an exception to the general rule to meet with

a limestone as pure as ordinary white chalk, or with clay as aluminous as that

used in Cornwall for porcelain, or with sand so entirely composed of siliceous

grains as the white sand of Alum Bay, in the Isle of Wight, employed in the

manufacture of glass, or sandstone so pure as the grit of Fontainebleau, used

for pavement in France. More commonly we find sand and clay, or clay and marl,

intermixed in the same mass. When the sand and clay are each in considerable

quantity, the mixture is called LOAM. If there is much calcareous matter in clay

it is called MARL; but this term has unfortunately been used so vaguely, as

often to be very ambiguous. It has been applied to substances in which there is

no lime; as, to that red loam usually called red marl in certain parts of

England. Agriculturists were in the habit of calling any soil a marl which, like

true marl, fell to pieces readily on exposure to the air. Hence arose the

confusion of using this name for soils which, consisting of loam, were easily

worked by the plough, though devoid of lime.

 

MARL SLATE bears the same relation to marl which shale bears to clay, being a

calcareous shale. It is very abundant in some countries, as in the Swiss Alps.

Argillaceous or marly limestone is also of common occurrence.

 

There are few other kinds of rock which enter so largely into the composition of

sedimentary strata as to make it necessary to dwell here on their characters. I

may, however, mention two others-- magnesian limestone or dolomite, and gypsum.

MAGNESIAN LIMESTONE is composed of carbonate of lime and carbonate of magnesia;

the proportion of the latter amounting in some cases to nearly one half. It

effervesces much more slowly and feebly with acids than common limestone. In

England this rock is generally of a yellowish colour; but it varies greatly in

mineralogical character, passing from an earthy state to a white compact stone

of great hardness. DOLOMITE, so common in many parts of Germany and France, is

also a variety of magnesian limestone, usually of a granular texture.

 

Gypsum is a rock composed of sulphuric acid, lime, and water. It is usually a

soft whitish-yellow rock, with a texture resembling that of loaf-sugar, but

sometimes it is entirely composed of lenticular crystals. It is insoluble in

acids, and does not effervesce like chalk and dolomite, because it does not

contain carbonic acid gas, or fixed air, the lime being already combined with

sulphuric acid, for which it has a stronger affinity than for any other.

Anhydrous gypsum is a rare variety, into which water does not enter as a

component part. GYPSEOUS MARL is a mixture of gypsum and marl. ALABASTER is a

granular and compact variety of gypsum found in masses large enough to be used

in sculpture and architecture. It is sometimes a pure snow-white substance, as

that of Volterra in Tuscany, well known as being carved for works of art in

Florence and Leghorn. It is a softer stone than marble, and more easily wrought.

 

FORMS OF STRATIFICATION.

 

A series of strata sometimes consists of one of the above rocks, sometimes of

two or more in alternating beds.

 

Thus, in the coal districts of England, for example, we often pass through

several beds of sandstone, some of finer, others of coarser grain, some white,

others of a dark colour, and below these, layers of shale and sandstone or beds

of shale, divisible into leaf-like laminae, and containing beautiful impressions

of plants. Then again we meet with beds of pure and impure coal, alternating

with shales and sandstones, and underneath the whole, perhaps, are calcareous

strata, or beds of limestone, filled with corals and marine shells, each bed

distinguishable from another by certain fossils, or by the abundance of

particular species of shells or zoophytes.

 

This alternation of different kinds of rock produces the most distinct

stratification; and we often find beds of limestone and marl, conglomerate and

sandstone, sand and clay, recurring again and again, in nearly regular order,

throughout a series of many hundred strata. The causes which may produce these

phenomena are various, and have been fully discussed in my treatise on the

modern changes of the earth's surface. (Consult Index to Principles of Geology,

"Stratification" "Currents" "Deltas" "Water" etc.) It is there seen that rivers

flowing into lakes and seas are charged with sediment, varying in quantity,

composition, colour, and grain according to the seasons; the waters are

sometimes flooded and rapid, at other periods low and feeble; different

tributaries, also, draining peculiar countries and soils, and therefore charged

with peculiar sediment, are swollen at distinct periods. It was also shown that

the waves of the sea and currents undermine the cliffs during wintry storms, and

sweep away the materials into the deep, after which a season of tranquillity

succeeds, when nothing but the finest mud is spread by the movements of the

ocean over the same submarine area.

 

It is not the object of the present work to give a description of these

operations, repeated as they are, year after year, and century after century;

but I may suggest an explanation of the manner in which some micaceous

sandstones have originated, namely, those in which we see innumerable thin

layers of mica dividing layers of fine quartzose sand. I observed the same

arrangement of materials in recent mud deposited in the estuary of Laroche St.

Bernard in Brittany, at the mouth of the Loire. The surrounding rocks are of

gneiss, which, by its waste, supplies the mud: when this dries at low water, it

is found to consist of brown laminated clay, divided by thin seams of mica. The

separation of the mica in this case, or in that of micaceous sandstones, may be

thus understood. If we take a handful of quartzose sand, mixed with mica, and

throw it into a clear running stream, we see the materials immediately sorted by

the water, the grains of quartz falling almost directly to the bottom, while the

plates of mica take a much longer time to reach the bottom, and are carried

farther down the stream. At the first instant the water is turbid, but

immediately after the flat surfaces of the plates of mica are seen all alone,

reflecting a silvery light, as they descend slowly, to form a distinct micaceous

lamina. The mica is the heavier mineral of the two; but it remains a longer time

suspended in the fluid, owing to its greater extent of surface. It is easy,

therefore, to perceive that where such mud is acted upon by a river or tidal

current, the thin plates of mica will be carried farther, and not deposited in

the same places as the grains of quartz; and since the force and velocity of the

stream varies from time to time, layers of mica or of sand will be thrown down

successively on the same area.

 

ORIGINAL HORIZONTALITY.

 

It is said generally that the upper and under surfaces of strata, or the "planes

of stratification," are parallel. Although this is not strictly true, they make

an approach to parallelism, for the same reason that sediment is usually

deposited at first in nearly horizontal layers. Such an arrangement can by no

means be attributed to an original evenness or horizontality in the bed of the

sea: for it is ascertained that in those places where no matter has been

recently deposited, the bottom of the ocean is often as uneven as that of the

dry land, having in like manner its hills, valleys, and ravines. Yet if the sea

should go down, or be removed from near the mouth of a large river where a delta

has been forming, we should see extensive plains of mud and sand laid dry,

which, to the eye, would appear perfectly level, although, in reality, they

would slope gently from the land towards the sea.

 

This tendency in newly-formed strata to assume a horizontal position arises

principally from the motion of the water, which forces along particles of sand

or mud at the bottom, and causes them to settle in hollows or depressions where

they are less exposed to the force of a current than when they are resting on

elevated points. The velocity of the current and the motion of the superficial

waves diminish from the surface downward, and are least in those depressions

where the water is deepest.

 

(FIGURE 1. Layers of sand and ashes on uneven ground.)

 

A good illustration of the principle here alluded to may be sometimes seen in

the neighbourhood of a volcano, when a section, whether natural or artificial,

has laid open to view a succession of various-coloured layers of sand and ashes,

which have fallen in showers upon uneven ground. Thus let A B (Figure 1) be two

ridges, with an intervening valley. These original inequalities of the surface

have been gradually effaced by beds of sand and ashes c, d, e, the surface at e

being quite level. It will be seen that, although the materials of the first

layers have accommodated themselves in a great degree to the shape of the ground

A B, yet each bed is thickest at the bottom. At first a great many particles

would be carried by their own gravity down the steep sides of A and B, and

others would afterwards be blown by the wind as they fell off the ridges, and

would settle in the hollow, which would thus become more and more effaced as the

strata accumulated from c to e. Now, water in motion can exert this levelling

power on similar materials more easily than air, for almost all stones lose in

water more than a third of the weight which they have in air, the specific

gravity of rocks being in general as 2 1/2 when compared to that of water, which

is estimated at 1. But the buoyancy of sand or mud would be still greater in the

sea, as the density of salt-water exceeds that of fresh.

 

(FIGURE 2. Section of strata of sandstone, grit, and conglomerate.)

 

Yet, however uniform and horizontal may be the surface of new deposits in

general, there are still many disturbing causes, such as eddies in the water,

and currents moving first in one and then in another direction, which frequently

cause irregularities. We may sometimes follow a bed of limestone, shale, or

sandstone, for a distance of many hundred yards continuously; but we generally

find at length that each individual stratum thins out, and allows the beds which

were previously above and below it to meet. If the materials are coarse, as in

grits and conglomerates, the same beds can rarely be traced many yards without

varying in size, and often coming to an end abruptly. (See Figure 2.)

 

DIAGONAL OR CROSS STRATIFICATION.

 

(FIGURE 3. Section of sand at Sandy Hill, near Biggleswade, Bedfordshire. Height

20 feet. (Green-sand formation.))

 

(FIGURE 4. Layers of sediment on a bank.)

 

(FIGURE 5. Nearly horizontal layers of sediment over sloping strata.)

 

(FIGURE 6. Cliff between mismer and Dunwich.)

 

There is also another phenomenon of frequent occurrence. We find a series of

larger strata, each of which is composed of a number of minor layers placed

obliquely to the general planes of stratification. To this diagonal arrangement

the name of "false or cross bedding" has been given. Thus in the section (Figure

3) we see seven or eight large beds of loose sand, yellow and brown, and the

lines a, b, c mark some of the principal planes of stratification, which are

nearly horizontal. But the greater part of the subordinate laminae do not

conform to these planes, but have often a steep slope, the inclination being

sometimes towards opposite points of the compass. When the sand is loose and

incoherent, as in the case here represented, the deviation from parallelism of

the slanting laminae can not possibly be accounted for by any rearrangement of

the particles acquired during the consolidation of the rock. In what manner,

then, can such irregularities be due to original deposition? We must suppose

that at the bottom of the sea, as well as in the beds of rivers, the motions of

waves, currents, and eddies often cause mud, sand, and gravel to be thrown down

in heaps on particular spots, instead of being spread out uniformly over a wide

area. Sometimes, when banks are thus formed, currents may cut passages through

them, just as a river forms its bed. Suppose the bank A (Figure 4) to be thus

formed with a steep sloping side, and, the water being in a tranquil state, the

layer of sediment No. 1 is thrown down upon it, conforming nearly to its

surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession,

so that the bank B C D is formed. If the current then increases in velocity, it

may cut away the upper portion of this mass down to the dotted line e, and

deposit the materials thus removed farther on, so as to form the layers 5, 6, 7,

8. We have now the bank B, C, D, E (Figure 5), of which the surface is almost

level, and on which the nearly horizontal layers, 9, 10, 11, may then

accumulate. It was shown in Figure 3 that the diagonal layers of successive

strata may sometimes have an opposite slope. This is well seen in some cliffs of

loose sand on the Suffolk coast. A portion of one of these is represented in

Figure 6, where the layers, of which there are about six in the thickness of an

inch, are composed of quartzose grains. This arrangement may have been due to

the altered direction of the tides and currents in the same place.

 

(FIGURE 7. Section from Monte Calvo to the sea by the valley of the Magnan, near

Nice.

A. Dolomite and sandstone. (Green-sand formation?)

a, b, d. Beds of gravel and sand.

c. Fine marl and sand of Ste. Madeleine, with marine (Pliocene) shells.)

 

The description above given of the slanting position of the minor layers

constituting a single stratum is in certain cases applicable on a much grander

scale to masses several hundred feet thick, and many miles in extent. A fine

example may be seen at the base of the Maritime Alps near Nice. The mountains

here terminate abruptly in the sea, so that a depth of one hundred fathoms is

often found within a stone's throw of the beach, and sometimes a depth of 3000

feet within half a mile. But at certain points, strata of sand, marl, or

conglomerate intervene between the shore and the mountains, as in the section

(Figure 7), where a vast succession of slanting beds of gravel and sand may be

traced from the sea to Monte Calvo, a distance of no less than nine miles in a

straight line. The dip of these beds is remarkably uniform, being always

southward or towards the Mediterranean, at an angle of about 25 degrees. They

are exposed to view in nearly vertical precipices, varying from 200 to 600 feet

in height, which bound the valley through which the river Magnan flows.

Although, in a general view, the strata appear to be parallel and uniform, they

are nevertheless found, when examined closely, to be wedge-shaped, and to thin

out when followed for a few hundred feet or yards, so that we may suppose them

to have been thrown down originally upon the side of a steep bank where a river

or Alpine torrent discharged itself into a deep and tranquil sea, and formed a

delta, which advanced gradually from the base of Monte Calvo to a distance of

nine miles from the original shore. If subsequently this part of the Alps and

bed of the sea were raised 700 feet, the delta may have emerged, a deep channel

may then have been cut through it by the river, and the coast may at the same time have acquired its present configuration.