THE FOSSILIFEROUS ROCKS. 15 
rare instances where volcanic ashes have enveloped animais 
or plants which were living in the sea or on the land in the 
immediate vicinity of the volcanic focus. The second great 
division of rocks is that of the Fossiliferous, Aqueous, or Sedi- 
mentary Rocks. These are formed at the surface of the earth, 
and, as implied by one of their names, are invariably deposited 
in water. They are produced by vital or chemical action, or 
are formed from the “sediment” produced by the disintegra- 
tion and reconstruction of previously existing rocks, without 
previous solution ; they mostly contain fossils; and they are 
arranged in distinct layers or “strata.” The so-called “aerial” 
rocks which, like beds of blown sand, have been formed by 
the action of the atmosphere, may also contain fossils; but 
they are not of such importance as to require special notice 
here. 
For all practical purposes, we may consider that the Aque- 
ous Rocks are the natural cemetery of the animals and plants 
of bygone ages; and it is therefore essential that the palezon- 
tological student should be acquainted with some of the prin- 
cipal facts as to their physical characters, their minute structure 
and mode of origin, their chief varieties, and their historical 
succession. . 
The Sedimentary or Fossiliferous Rocks form the greater 
portion of that part of the earth’s crust which is open to our 
examination, and are distinguished by the fact that they are 
regularly “stratified” or arranged in distinct and definite layers 
or “strata.” These layers may consist of a single material, 
as in a block of sandstone, or they may consist of different 
materials. When examined on a large scale, they are always 
found to consist of alternations of layers of different mineral 
composition. We may examine any given area, and find in it 
nothing but one kind of rock—sandstone, perhaps, or lime- 
stone. In all cases, however, if we extend our examination 
sufficiently far, we shall ultimately come upon different rocks; 
and, as a general rule, the thickness of any particular set of 
beds is comparatively small, so that different kinds of rock 
alternate with one another in comparatively small spaces. 
As regards the origin of the Sedimentary Rocks, they are 
for the most part “ derivative” rocks, being derived from the 
wear and tear of pre-existent rocks. Sometimes, however, they 
owe their origin to chemical or vital action, when they would 
more properly be spoken of simply as Aqueous Rocks. As to 
their mode of deposition, we are enabled to infer that the 
materials which compose them have formerly been spread out 
by the action of water, from what we see going on every day 
