SECTION VI.

GEOLOGY, MINERALOGY, AND CHEMISTRY.

AIM AND METHODS OF THE SCIENCE OF GEOLOGY.

(From Page's "Advanced Text-Book of Geology.")

GEOLOGY (from the Greek words ge, the earth, and logos, discourse or reasoning) may be defined as that department of natural science which treats of the mineral structure of our globe. Its object is to examine the various materials of which our planet is composed, to describe their appearance and relative positions, to investigate their nature and mode of formation, and generally to discover the laws which seem to regulate their arrangement. Being unable to penetrate beyond a few thousand feet into the solid substance of the earth, the researches of geologists are necessarily limited to its exterior shell or crust; hence they speak of the "crust of the globe,” meaning thereby that portion of the rocky structure accessible to human investigation. Speculations as to the nature of the interior, as bearing on scientific problems, are no doubt permissible, and, aided by astronomical data, we may ascertain the bulk, density, and other conditions of the mass; but all this must be carefully separated from geological deductions, which are based on absolute facts and known appearances. The geologist has thus a clear and unmistakable course before him ; his duty is to observe, examine, and compare, to ascend from a knowledge of facts to a consideration of the laws by which they are governed; and thus endeavour to unfold, as far as human reason can, the history of the marvellous planet he inhabits.

The materials composing the earth's crust are rocks of various kinds as granite, roofing-slate, marble, sandstone, coal, chalk, clay, and sand-some hard and compact, others soft and incohering. These substances do not occur indiscriminately in every part of the world, nor, when found, do they always lie

in the same positions. Granite, for example, may exist in one district of a country, roofing-slate in another, coal in a third, and chalk in a fourth. Some of these rocks occur in irregular mountain-masses, while others are spread out in regular layers or courses, termed strata, from the Latin word stratum, strewn or spread out. It is evident that substances differing so widely in composition and structure must have been formed under different circumstances, and by different causes; and it becomes the province of the geologist to discover those causes, and thus infer the general conditions of the regions in which, and of the periods when, such different rock-substances were produced.

When we sink a well, for instance, and dig through certain elays, sands, and gravels, and find them succeeding each other in layers, we are instantly reminded of the operations of water, seeing it is only by such agency that accumulations of clay, sand, and gravel are formed at the present day. We are thus led to inquire as to the origin of the materials through which we dig, and to discover whether they were originally deposited in river courses, in lakes, in estuaries, or along the sea-shore. In our investigation we may also detect shells, bones, and fragments of plants imbedded in the clays and sands; and thus we have a further clue to the history of the strata through which we pass, according as the shells and bones are the remains of animals that lived in fresh-water lakes and rivers, or inhabited the waters of the ocean. Again, in making a railway cutting, excavating a tunnel, or sinking a coal-pit, we may pass through many successions of strata-such as clay, sandstone, coal, limestone, and the like; and each succession of strata may contain the remains or impressions of different plants and animals. Such differences can only be accounted for by supposing each stratum or set of strata to have been formed by different agencies and in different localities,-under different conditions of climate and under varying arrangements of sea and land, just as at the present day the rivers, estuaries, and seas of different countries are characterised by their own special accumulations and the imbedded remains of their own peculiar plants and animals.

In making these investigations the geologist is guided by his knowledge of what is now taking place on the surface of the globe-reasoning from the known to the unknown, and ascribing similar results to similar or analogous causes. Thus, at the present day, we see rivers carrying down mud and sand

and gravel, and depositing these in layers, either in lakes, in estuaries, or along the bottom of the ocean. By this process many lakes and estuaries have, within a comparatively recent period, been filled up and converted into dry land. We see also the tides and waves wasting away the sea-cliffs in one district, and accumulating wide tracts of sand and gravel in bays and other sheltered recesses. By this process thousands of acres of land have been washed away and covered by the sea, even within the memory of man; while by the same means new tracts have been formed in districts formerly covered by the tides and waves. Further, we learn that, during earthquake convulsions, large districts of country have sunk beneath the waters of the ocean; while in other regions the sea-bottom has been elevated into dry land. Volcanic action is also sensibly affecting the surface of the globe-converting level tracts into mountain ridges, throwing up new islands from the sea, and casting forth molten lava and other materials, which in time become hard and consolidated rock-masses.

Now, as these and other agents are at present modifying the surface of the globe, and changing the relative positions of sea and land, so in all time past have they exerted a similar influence, and have necessarily been the main agents employed in the formation of the rocky crust which it is the province of geology to investigate. Not a foot of the land we now inhabit but has been repeatedly under the ocean, and the bed of the ocean has formed as repeatedly the habitable dry land. No matter how far inland, or at what elevation above the sea, we now find accumulations of sand and gravel,—no matter at what depth we discover strata of sandstone or limestone,—we know, from their composition and arrangement, that they must have been formed under water, and been brought together by the operations of water, just as layers of sand and gravel and mud are accumulated or deposited at the present day. And as earthquakes and volcanoes break up, elevate, and derange the present dry land-here sinking one portion, there tilting up another, and everywhere producing rents and fissures: so must the fractures, derangements, and upheavals among the strata of the rocky crust be ascribed to the operation of similar agents in remote and distant epochs.

By the study of existing operations, we thus get a clue to the geological history of the globe; and the task is rendered much more definite and certain by an examination of the plants

and animals found imbedded in the various strata. At present, shells, fishes, and other animals are buried in the mud or silt of lakes and estuaries; rivers also carry down the carcasses of land animals, the trunks of trees and other vegetable drift; and earthquakes submerge plains and islands, with all their vegetable and animal inhabitants. These remains become enveloped in the layers of mud and sand and gravel formed by the waters, and in process of time are petrified (petra a stone, and fio I become); that is, are converted into stony matter like the shells and bones found in the oldest strata. Now, as at present so in all former time must the remains of plants and animals have been similarly preserved; and as one tribe of plants is peculiar to the dry plain, and another to the swampy morass,family belongs to a temperate, and another to a tropical region,-so, from the character of the imbedded plants, are we enabled to arrive at some knowledge of the conditions under which they flourished. In the same manner with animals: each tribe has its locality assigned it by peculiarities of food, climate, and the like; and by comparing fossil remains (fossil, from fossus, dug up; applied to all remains of plants and animals imbedded in the rocky crust) with existing races, we are enabled to determine many of the past conditions of the world with considerable certainty.

THE YOUNG GEOLOGIST.

-as one

(From "The Old Red Sandstone," by HUGH MILLER.)
Quartz, n. a German miner's term
for crystallized silica; rock-
crystal.

Di-lu'vi-um, n. (L. dis, luo), the
term usually applied to matter
brought together by the extra-
ordinary action of water, as in
the Noachian deluge,-the term
allu'vium being restricted to
accumulations of earth, clay,
sand, gravel &c., by rivers,
floods, and the ordinary agency
of water.
Granite, n. (L. granum), literally,
grain-stone, an aggregate of
felspar, quartz, and mica, or of
two at least of these minerals. It
is an igneous or unstratified rock,
and is considered the foundation
rock of the globe, or that upon
which all secondary rocks re-
pose.

Gneiss, n. (rīce), a species of
aggregated rock, composed of
quartz, felspar, and mica, and
of a structure more
or less
laminated or slaty. It is rich
in metallic ores, but contains
no fossil remains.
Horn'blende, n. (Ger. blenden, to
dazzle), a simple mineral of fre-
quent occurrence in granite and
trappean rocks; so called from
its horn-like cleavage, and pe-
culiar lustre. It is usually of a
dark-green or black colour.
Shale, n. (Ger. schalen, to peel off),

an indurated or hardened slaty

THE gunpowder having loosened a large mass in one of the inferior strata, our first employment, on resuming our labours, was to raise it from its bed. I assisted the other workmen in placing it on edge, and was much struck by the appearance of the platform on which it had rested. The entire surface was ridged and furrowed like a bank of sand that had been left by the tide an hour before. I could trace every bend and curvature, every cross hollow and counter ridge of the corresponding phenomena; for the resemblance was no half resemblance-it was the thing itself; and I had observed it a hundred and a