CLASSIFICATION OF ROCKS. 
37 
A physical system was then established, and to this system organic beings were to be 
adapted. There were controlling agents. Of these the atmosphere was one, and caloric 
another; and these have continued and will continue to control the types of organization 
to the end of time. Vary the present standard, if only in a narrow compass, and but few 
if any of the present races would continue to exist. 
In view of this subject, I hazard the assertion that the composition of the atmosphere 
was never essentially different since the JYereites of the Taconic system were created; and 
also that the temperature has never been greater than it is now, since that period. This 
is going back as far as it is possible with organic beings: none older are now known to 
exist. Because a lizard or crocodile does not consume so much oxygen as an ox in a given 
period, it does not follow that in the era of the Lias, an era of lizards, the atmosphere 
contained less oxygen or more carbonic acid than it does now; for with their respiratory 
apparatus, we have a right to infer that if the proportion of oxygen was less than it is at 
present, they would not be supplied with that material, and enough could not be obtained 
if less existed in the atmosphere. When we speak, therefore, of the changes which usher 
in a new system, it is not intended to inculcate the doctrine that they were so great, or of 
such a character, as would be incompatible with the present; or that organic beings would 
be unfitted organically for any other period or era in the world’s history. 
Systems are subdivided into groups; the groups holding the same relation to a system, 
as the system to the totality of the consolidated sediments. The beginning and end of a 
group is marked by some important change, such as the disappearance of affiliated tribes 
and species. It is then by observations of this kind, that divisions and subdivisions of the 
sediments are obtained. Names which are supposed to be appropriate at the time, are 
conferred upon the systems and groups. They may subsequently, however, be demon¬ 
strated to be inappropriate; the progress of discovery outgrowing and thereby rendering 
obsolete the nomenclature. This is an evil; and one who is disposed to cavil, might lay 
hold of the fact to the prejudice of the science of geology, on the ground that nothing is 
settled ; that it is a subject of opinions and speculations, and not of facts and principles ; 
of endless details and fanciful hypotheses, which every man has a right to invent for his 
own or his neighbor’s amusement. But such cavillers belong to a race too lazy to observe, 
too self-conceited to profit by facts, or too bigoted to look at truth when they fear it may 
conflict with their own notions. They are too obstinate to be reformed ; and if they were 
reformed, they would be of little use to science in any of its departments. 
The Primary rocks, comprehending granite, hypersthene, primary limestone, serpen¬ 
tine, gneiss, mica and talcose slates, hornblende, sienite, trap and greenstone, require our 
attention first of all. They may be tabulated as follows : 
