MUSEUM OF COMPARATIVE ZOOLOGY, 277 
No natural distinction can be drawn between rocks of the Tertiary 
and Pre-Tertiary ages, since the glass and fluidal inclusions, crystalline 
texture, and the various other characters fail, exactly where they are 
most needed, to divide the rocks into older and younger, as is done 
by the majority of lithologists. 
The writer believes that rocks should be studied, by beginning with 
their most compact or glassy state, and by then tracing them through 
to the most crystalline form, following every alteration, whether it be 
chemical or mechanical. Every rock that can be traced in this way 
forms a distinct species, whatever may be its state, — whether amorphous, 
glassy, crystalline, fragmental, tufaceous, or otherwise, — and whatever 
may be its age. The modifications, if of sufficient importance, form 
varieties simply, which should be included under the specific name. A 
natural classification of rocks must be empirical, and must be based on 
the rock as a whole, while a natural mineralogical classification is an 
impossibility, as it is based on part of the characters only, 
If we except the veinstones and the majority of those rocks that are 
composed of one mineral, the species of rock forming the crust of the globe 
are very few. Believing that this earth is a cooling globe, all mani- 
festations of internal heat giving rise to rocks (the only thing with which 
we are at present concerned) are here termed volcanic, and all such pro- 
ducts are styled volcanic rocks. The testimony of the rocks is that all 
sedimentary forms came primarily from volcanic ones, volcanic energy 
having been more active than now in the past ages of the globe. This 
derivation is consonant with that which we sce taking place at the 
present time, and agrees with the law of dissipation of energy ; while the 
reverse view, at present popular, — that eruptive rocks were derived 
from sedimentary ones, — is contrary to the positive testimony of the 
rocks themselves, to the facts that are observed in nature, and to physi- 
cal laws. 
Taking the consolidation of any rock as its initial point, the minerals 
and rock fragments contained therein fall naturally into three classes : 
1. Minerals and fragments of prior origin ; 2. The products of that con- 
solidation ; 3. The products of alteration and infiltration. 
These three classes are most marked in the volcanic rocks, as is 
natural ; the first two predominating in the younger and least altered, 
the latter in the older and more altered ones, while the first and third 
classes predominate in sedimentary rocks. These alterations apparently 
take place through the agency of the ordinary percolating waters, which 
are not necessarily hot. The minerals and, fragments of the first class, 
