136 GEOLOGY. 
botany, mineralogy, and geography of the ancient 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 
understand certain things regarding their kinds, structure, and arrange- 
ment. 
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 ponsideiible 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 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- SARS 
Kinds of Rocks.—Rocks have different names, “poanediee to their 
appearance and structure. Every one knows what sand is, and that 
it varies greatly in fineness. The most of the sand we see is com- 
posed of small particles of rock ground to powder, but it often 
consists, as we shall afterwards learn, of numberless very minute 
shells. Sandstone is the usual rock of which houses are built, and 
which, in thin layers, is used for pavement. 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 have 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 ball-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 clay, 

