GEOLOGY. 



NATURE OF THE SUBJECT. 



WE have most of us stood at the base of a 

 great cliff, and looked upwards with awe 

 at the rocks exposed on its weathered front. Such 

 a sight might suggest many strange and interesting 

 inquiries. How did these rocks come to be where 

 they are? Of what are they composed? When 

 were they formed ? Whence the material for the 

 vast thickness of rock that composes the crust of 

 the earth ? Whence have come the varied sub- 

 stances that form our limestones, coals, and sand- 

 stones ? Whence also the strange shells, plants, 

 and animals that a closer examination of their 

 structure reveals? Are these the remains of 

 bygone living organisms, or are they only marks 

 in the rocks themselves ? If they were once living 

 creatures, what were their structure and habits? 

 Such questions suggest themselves to every think- 

 ing person, and such questions Geology undertakes 

 to answer. 



Geology, from the Greek ge, the earth, and logos, 

 a description, is, according to its name, a descrip- 

 tion of the earth. It examines the various rocks 

 that compose its crust, and seeks to explain their 

 appearance, form, structure, relative position, for- 

 mation, age, and distribution throughout the globe. 

 It also inquires minutely into their contents, 

 animal, vegetable, and physical ; the causes of 

 their imprisonment in their stony tombs ; and the 

 structure and habits of the creatures there found. 

 It pictures forth the physical history of the globe 

 during the successive epochs through which it has 

 passed, with their varied scenery and inhabitants, 

 the formation of its many strata, and the structure 

 and progress of the organic forms that successively 

 waved in its atmosphere, moved over its surface, 

 or swam in its seas. In short, it is the province 

 of geology to describe the whole natural history 

 of the globe during the various ages of the long 

 past ; and it includes the ancient zoology, botany, 

 mineralogy, and geography of the earth, whose 

 present conditions are the result of the numberless 

 changes through which it has passed in these 

 geological eras. The past it seeks to interpret 

 solely by the present, assured that the laws of 

 nature are invariable and universal, and that 

 causes operating now produced like effects in the 

 primeval earth. 



ROCKS, THEIR KINDS, STRUCTURE, AND 

 DISPOSITION. 



In order to speak with precision in our study 

 of this subject, it is necessary to have a distinct 

 idea of what a rock is in geology, and to under- 

 stand certain things regarding the kinds, structure, 

 and arrangement of rocks. 



What a Rock is in Geology. In geology, the 

 word rock has a wider meaning than it has in 

 common language, where it means a mass of stone 

 of considerable size. In this science, the word 

 rock is used to designate any of the materials 

 that compose the crust of the earth, of whatever 

 size and softness they may be. Geologists reckon 

 2 



sandstone, marble, quartz, granite, and limestone 

 to be rocks, as others do ; but they also speak 

 of coal, gravel, chalk, sand, salt, peat, and like 

 soft and broken substances, as rocks or rock- 

 'ormations. 



Kinds of Rocks. Rocks have different names, 

 according to their appearance and structure. 

 Every one knows what sand is, and that it varies 

 jreatly in fineness. The most of the sand we see 

 s composed of small particles of rock ground to 

 sowder, but it often consists, as we shall after- 

 wards learn, of numberless very minute shells. 

 Sandstone is the usual rock of which houses are 

 auilt, and which, in thin layers, is used for pave- 

 ment. This rock is more common than any other, 

 and has many varieties, and is, of course, so called 

 because it is composed of particles of sand that 

 tiave been made to cohere. When the particles 

 of the sandstone are somewhat larger and sharper, 

 the rock is called grit, from the particles having 

 been grated down or broken : the rock of which 

 millstones are formed is called millstone-grit, and 

 its value depends on the hardness and sharpness 

 of the grains of which it is composed. When the 

 particles are larger still, and form small stones 

 that do not cohere, the rock is called gravel; and 

 when yet larger and more rounded, shingle, 

 examples of both of which occur on the sea-beach. 

 A mass of broken angular stones thrown up in a 

 heap, as by a river after a flood, is called rubble. 

 A stone when small is called a. pebble; when large, 

 a block; and when rounded and worn, a boulder, 

 because it is ^//-shaped. The fine sediment at 

 the bottoms of rivers, lakes, and pools is composed 

 of ground mineral, animal, and vegetable matter. 

 When this is tough and plastic, it is called cloy, 

 because it cleaves or sticks ; and the whole accumu- 

 lation of mud, clay, and sand at the bottom of any 

 water, is called silt. 



The remains of vegetable matter found in various 

 parts of the country, and used as fuel, are known 

 as peat; and coal is nothing but such vegetable 

 matter changed by heat, and hardened into rock 

 by pressure. Limestone is the name given to the 

 hard rock which, after being burned in a kiln, 

 forms lime. When the limestone is hard and 

 crystalline, it forms marble, which is of different 

 colours, from deep black to pure white, and often 

 beautifully variegated. Chalk is a variety of lime- 

 stone, and obtains its name from this fact ; the 

 word chalk being another form of the Latin calx, 

 lime. 



Common slate, used for writing on and for 

 roofing, is composed of thin layers of hard rock, 

 of which some of our highest mountains are formed. 

 The name shale is applied to a kind of rock which 

 shells off or splits into very thin layers, and which 

 may be seen in great heaps near coal-pits. Thin 

 layers of sandstone used for pavement are called 

 flags. The white pebbles so common on the sea- 

 beach, and so easily broken, are made of quartz, 

 and rock formed of it is called quartz-rock. Some 

 varieties of quartz, called rock-crystals, are very 

 beautiful and valuable, and are reckoned precious 

 stones, such as agate, amethyst, and topaz. 



