18 PRIMARY CLASSIFICATION OP THE ROCKS. 
PRIMARY CLASSIFICATION OF THE ROCKS. 
10. All the varieties of rock may be separated into two great 
classes. The members of one class agree in having a crystalline 
Structure, more or less perfect, indicating that they have once 
been in a liquid state, and that their .particles have been united 
by a crystallization that is confused, and partial, by reason of the 
interference of one crystalline form with another. The members 
of the other class are sometimes made up almost exclusively of 
the exuviae of shell-fish ; but more commonly they are composed 
of rounded pebbles, sand, and clay, that have proceeded from the 
destruction of the older rocks, by different agents, and especially 
by water, with only a few organic remains, occurring at distant 
intervals, imbedded. These materials have been collected and 
consolidated into masses, having generally, though not always, a 
texture and aspect which betray their mechanical origin. 
There is nothing in the appearances presented by these two 
kinds of rock, to indicate that they do not alike descend to great 
depths, and even penetrate quite to the centre of the earth. But 
the operations of mining ; the scooping out of vallies in the solid 
body of the continents, by causes that are now in operation, or that 
have been active in former times ; and observations on the posi- 
tion of the place of separation between them, have brought man- 
kind acquainted with the structure of the crust of the globe, and 
of the mineral masses that compose it. The members of the 
second of these classes are found to form only a superficial cover- 
ing upon the top of the others ; the depth to which they descend 
being, (at least when compared with the whole radius of the 
earth), but small. 
From their position, resting upon and covering the others, it 
has been inferred that they are of more recent origin ; that the 
crystalline rocks were first formed ; that their upper portions have 
been broken in pieces by the waves and other disintegrating 
agents, rolled upon each other, and reduced either to small frag- 
ments or to powder, and afterwards collected into beds and con- 
solidated, forming the class of which we are now speaking. The 
crystalline are therefore called primitive, and the fragmented, or 
those having a mechanical origin, secondary rocks. Amongst 
the latter, Geologists have found it convenient to introduce cer- 
tain subdivisions, as the Transition, Proper Secondary, Tertiary 
and Overlying or Volcanic, which will be noticed more particu- 
larly hereafter. 
It is perhaps true, that neither the composition and structure, 
nor the situation of the secondary rocks, nor both taken in con- 
nexion, would Be -perfectly decisive of their mechanical origin, 
and of their having been formed subsequently to the others, if they 
contained no organic remains. These remains are not found in 
all the members of the class, but they pervade them so generally, 
