CAMBRIDGE ESSAYS I IIOPKTNs' ''gKOLOGy/^ 
427 
has been, and is still, departing from some primitive state widely dif- 
ferent from its actual one, and approaching to some ultimate condition 
to which it can only attain, under the action of recognized phj sical 
causes, after an indefinite period of time, though its approximation to 
that ultimate condition not only is now, but has been for an enormous 
period of time, so near as to admit of no change the rate of which can 
be deemed perceptible." 
In the early stages of a science, the observed phenomena are little 
more than so many insulated facts, but with accumulated observations 
there arises a necessity for classification. The phenomena thus 
classified are suggestive of hypotheses as to the physical causes to which 
the phenomena are due, and hence the science becomes inductive. 
The subject is, then, thus divided : — 
1. Descriptive Geoxogt. — The description of the observed pheno- 
mena of inorganic matter, and this arrangement into groups exhibiting 
the laws by which they may be connected. 
2. Palaeontology. — The branch which treats of the phenomena of 
organic matter, and of the stratigraphical and geographical distribution 
of fossil organic remains. 
3. Physical Geology, which comprises all those investigations and 
discussions by which we endeavour to connect the phenomena with the 
physical causes to which they are due. 
In accordance with this plan, Mr. Hopkins now proceeds to state the 
broad generalizations of this science and the conclusions immediately 
dcducible from them, restricting himself to those points, whether facts 
or theories, which are regarded as firmly and finally established ; and 
afterwards discussing some questions of high speculative interest, on 
which there still remains room for differences of opinion. The first fact 
to which attention is directed is " the division of the superficial portion 
of the earth's crust into two distinct portions, stratified and ua- 
Btratified. 
"The unstratified portion of the earth's crust is distinguished by the 
total absence of all stratification. In some cases there are planes of 
separation, termed divisional or structural planes, which might, at first 
sight, deceive an inexperienced eye, but which are soon seen to be 
totally distinct from planes of stratification. Its calcareous, siliceous, 
and argillaceous constituents exist much more in mineralogical combi- 
nation, the first being in much smaller proportion, and the general 
structure is far more crystalline than in the stratified mass. Granite 
may be instanced as one familiar form of unstratified rock. 
"But there is a far more important and significant distinction be- 
tween these stratified and unstratified masses. The former contains 
within it, imbedded in almost every stratum, the remains of organic 
objects, animals and plants ; whereas no trace of such objects is found 
in the unstratified mass. They comprise great numbers of bones of 
vertebrate, and shells of invertebrate animals, with numerous remains 
of zoophytes. The great majority of these remains are completely 
mineralized, every particle of animal or vegetable matter having entirely 
disappeared, and been replaced by mineral substances. It is beautiful 
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