Composition^ and Analogies of Rocks.'iil 01 



considered as most essential, it is interesting to observe how few 

 are the rocks which are produced from them. If the varieties 

 are most numerous in the primary or older series, they are still 

 few, and, within certain limits of variation, very constant. In 

 the later rocks, the varieties are still more limited ; and, when we 

 reflect on the circumstances under which they have been pro- 

 duced, they are confined to a much less number than could have 

 been expected. As most of the minerals of ordinary occurrence 

 are formed, for example, of the earths which exist in granite 

 and gneiss, why might we not expect to find garnet, corundum, 

 or andalusite, in every one ; instead of being, as they are, limited, 

 to a few occasional specimens. We are equally at a loss to ac- 

 count for those distinctions between gneiss, micaceous schist, 

 quartz rock, or other substances, that occur in the same ancient 

 series ; distinctions which, on the great scale, are really steady 

 and definite, notwithstanding the occasional interferences of 

 character that occur in particular cases. That these have been 

 regulated by certain chemical laws, is unquestionable, however 

 incomprehensible the nature of these may be. It must also be 

 from this cause, that such rocks are found to preserve the same 

 characters, wherever they occur ; a circumstance otherwise cal- 

 culated to excite our surprise. In every other department of 

 nature, her productions vary according to the climate and situa- 

 tion, but granite is the same in Egypt and in Greenland. It is 

 with the laws of organization alone that climate interferes. 



As the secondary, or later, strata have been chiefly formed 

 from the waste of the ancient rocks, it is less surprising that they 

 should preserve a general constancy of character throughout the 

 globe, however individuals may vary in different places. Even 

 these variations are still remarkable ; as well from their steadi- 

 ness, as from the extent through which that uniformity can some- 

 times be traced. The difference between compact limestone and 

 chalk, is no less remarkable than the similarity which, in dis- 

 tant places, occurs between strata that we can scarcely conceive 

 to have formed parts of one deposit. It is worthy of remark, 

 however, that, in the secondary strata, the most conspicuous 



