Trof. T. Sterry Hunt — Elements of Primary Geology. 495 



alumina and alkalies in the masses. In the study of these rocks 

 as members of great succeeding groups, it is however necessary to 

 take account of the products, alike soluble and insoluble, of the 

 subaerial decay of previously exposed masses, both crenitic and 

 plutonic, of the frequent intervention of sea- water bringing mag- 

 nesian salts, and also of the direct and indirect intervention of 

 products of organic life ; all of which have caused at different 

 periods variations in the constitution of the rock-masses generated. 

 From these considerations it will be apparent that no series of 

 deposits subsequent to the ancient granitoid gneisses can ever have 

 possessed the same degree of uniformity and of universality as they. 

 Variations from causes already assigned appear also in the constitu- 

 tion of the plutonic rocks of successive ages. 



5. Types of silico-aluminous crenitic rocks which are rare at a 

 given period become more abundant in a later time, and subsequently 

 disappear. The earlier types recur locally and less well character- 

 ized in later ages. From this it results that in the mineral as in 

 the organic world, generalizations as to chronological classification 

 must be based upon the mineral characters of the groiap considered 

 as a whole, and not on those of individual varieties. Eocks not 

 including aluminous silicates, — such as quartz, carbonate of lime, 

 iron-oxyds and protoxyd silicates, — recur with slight variations at 

 very widely separated periods in geologic time. 



6. The operations of solution and deposition, and those of sub- 

 aerial decay and disintegration during the earlier ages went on 

 under geographic conditions not very much unlike those of later 

 periods, so that in the great series of crenitic rocks certain groups 

 are often absent ; in some cases from non-deposition, and in others 

 from subsequent erosion connected with movements of elevation and 

 depression. From these causes, as well as from the universal con- 

 traction of the underlying plutonic stratum, due to the crenitic 

 process, stratigraphical breaks and discordances exist among the 

 crenitic strata as among the sedimentary rocks of later ages. All 

 such phenomena in stratigraphy, at whatever period recurring, are 

 however but local and accidental interruptions of the normal order 

 of mineralogical development. 



7. The great groups of stratiform crystalline rocks, which every- 

 where appear as the substratum of the uncrystalline sediments, 

 are essentially neptunian or crenitic in origin, and the few plutonic 

 masses of pre-Palgeozoic age have no other significance in the litho- 

 logical history of these groups than that which comes from the fact 

 that portions of these plutonic masses, or the results of their 

 subaerial decay, occasionally intervene in the formation of the 

 crenitic rocks. Their importance as factors in primary geology 

 is so small that their exclusion therefrom by Werner and his school 

 was an error much less grave than those of the endoplutonic and 

 exoplutonic schools, and especially of the latter; which, in its extreme 

 and logical form, assigns an eruptive origin to the whole category 

 of crenitic rocks, including gneisses, norites, steatites and serpentines, 

 and even quartzite, iron-oxyds and limestones. 



