CLASSIFICATION OF ROCKS. 37 



A pliysical system was then estalilislictl, and to this system organic bcinjis wore to he 

 adapted. There were controlliiii^ agents. Of these the atmosphere was one, and caloric 

 another; and these liave 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 i)Ut few 

 if any of the present races would continue to exist. 



In view of this suhjcct, I hazard the assertion that the composition of the atmosphere 

 was never essentially dillerent since the JVereites 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 jiossible 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 docs 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 tlie 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 afliliated 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 tlie ground that nothing is 

 settled ; that it is a subject of opinions and speculations, and not of facts and princijjles ; 

 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, rerpiire our 

 attention first of all. They may be tabulated as follows : 



