40 THE CANADIAN NATURALIST. [Vol. ix. 



ON SOME POINTS IN LITHOLOGY. 



By Prof. James D. Dana. 

 (^From the American Journal of Science.) 



I. On some of the characters employed in distinguishing 



DIFFERENT KINDS OF RoCKS. 



Lithology is a department of G-eology, rocks being the material 

 in and through which geological problems are presented for 

 study. The true aim of the science of lithology is to describe 

 the kinds of rocks mineralogically and chemically, and to note 

 down their distinctions in such a manner as shall best contribute 

 to the objects of geology ; and these latter objects include, as 

 regards rocks, the origin of the minerals and mineral associations, 

 constituting or occurring in rocks ; the origin of the rock masses 

 and their relations to other geological phenomena; and the 

 origin of all changes or transformations that have taken place in 

 rocks in the course of the earth's physical development. Geology, 

 chemistry and mineralogy have each to be considered in deter- 

 mining the proper distinctions between the kinds of rocks. 

 Should lithology make much of mere difference in texture, or in 

 ingredients that are present only in minute proportion, geology 

 might rightly say that, for such a purpose, these points are of 

 small importance compared with the nature or composition of 

 the mass. 



The defining of rocks is attended with special difficulties on 

 account of their mutual transitions. From granite down they 

 are, with very few exceptions, mixtures of minerals, as much so 

 as the mud of a mud bank. They graduate into one another by 

 indefinite blendings, as the mud of one mud bank graduates into 

 the mud of others around it. In fact a large part of the crys- 

 talline rocks were once actual mud beds or sand beds ; and even 

 part of the eruptive rocks may have been so in their earlier 

 history. Strongly drawn limits nowhere exist. Rocks are hence 

 of different kinds, not of different species; and only those mix- 

 tures are to be res-arded as distinct kinds of rocks which have a 

 sufficiently wide distribution to make a name important to the 

 geologist. Other kinds have to be classed as varieties, if worthy 

 of that degree of recognition. 



