148 The Geognosy of Crystalline Rocks. 



time penetrated by water, and has furnished directly the chief* 

 part of the basic eruptive rocks of various geological ages. This 

 primary plutonic stratum was considered as modified from with- 

 out by additions and subtractions through permeating waters, and. 

 also interiorly by differentiations, through a process of segregation, 

 which has probably been at work from a remote period. This 

 was illustrated by the slow crystallization from fused silicated 

 mixtures of species such as chrysolite and magnetite, which are 

 heavier and less fusible than the parent mass. From such par- 

 tially crystallized mixtures, a separation by eliquation may take 

 place ; and thus unlike, and more or less basic, portions may be 

 derived from a once-homogeneous plutonic mass. Evidences of" 

 some such process are afforded in the variations to be seen in 

 undoubtedly exotic basic rocks, which often present mineralogical 

 differences in contiguous portions, having a stratiform arrangement 

 due to movements of flow in extravasation. Phenomena of this 

 kind, which the author had formerly ascribed to the partial inter- 

 mingling by fusion of masses originally distinct, he now conceives 

 to be due to partial separation by the above-described process,, 

 the importance of which, in the history of igneous rocks, was long, 

 since recognized by Durocher. 



The limitations of such a process of differentiation by crystal- 

 lization and eliquation, and the distinction to be drawn between 

 exotic masses, occasionally stratiform in their internal arrangement, 

 and aggregates — whether in veins or in beds — of direct crenitic 

 origin, was insisted upon. The great extent and the importance 

 of certain crenitic masses deposited in fissures as veinstones, and. 

 often distinctly stratified, was noted, and it was maintained that a. 

 neglect to distinguish between these and exotic rocks has been the 

 source of much error and confusion among observers, who, by 

 confounding two kinds of mineral masses of unlike origin, have 

 obscured instead of elucidating important geological problems^ 

 The study of such masses belongs primarily to the chemist and 

 the mineralogist. 



