284 L. V. Pirsson — Classification of Igneous Rocks. 



of treatment they are not subdivided into groups in this 

 book but are considered in a section by themselves, as 

 will appear later. 



It is obvious that the character of rocks, which texture 

 affords, might be employed for classification at the very 

 beginning, or after the rocks have been separated by 

 mineral grouping. In either case the final result would 

 be the same, so far as classifying and naming the rock is 

 concerned. In the first case, if rocks are first divided 

 into granular, porphyritic, and dense classes and then 

 subdivided on mineral composition, we should have three 

 such tables as No. 2 to refer to; whereas, if the texture 

 is applied after the mineral grouping, the whole can be 

 condensed into one table, as has been done, with greater 

 convenience of reference. 



It is not, however, a necessity that we should follow the 

 latter ..plan in describing the rocks for the benefit of the 

 student. It is very much easier and more logical for him 

 to take up the coarser-grained rocks first; he can learn 

 to make his determinations of the minerals, and their 

 groupings and relative quantities and relations, which 

 settle the classification, much more quickly and accur- 

 ately, with them, and the knowledge thus gained can then 

 be applied efficiently to the more difficult fine-grained and 

 dense rocks. Moreover, in this one follows the usual 

 mental course of procedure in determining a rock: we 

 first examine the rock megascopically and notice whether 

 it is sufficiently coarse-grained to permit us to recognize 

 the component minerals ; we study them and then assign 

 the rock to its proper position in the scheme of classifica- 

 tion we have in mind. 



