e£sumi5 and genehal discussion. 549 



"only universal formation." He further remarks, that the Archfeau 

 rocks " extended over the whole globe, and were the floor of tlie ocean 

 and the material of all emerged land when life first began to exist." 

 Moreover, he expressly enumerates among the occurrences of Archajan 

 rock those areas of the earth's crust " which, in the course of tlie 

 upturnings of mountain-making, have been pushed upward among tlie 

 displaced strata, and in this way have been brought out to the light." 



Thus, according to Dana's definition of the term Archaean, it would 

 be legitimate to include under that designation the granitic axis of the 

 Sierra Nevada, although that mass of rock seems clearly to have as- 

 sumed its present position at some time near the close of the Jurassic 

 epoch. In fact we do not see how it would be possible not to admit that 

 ^tna, for instance, belongs to the Archaean, as defined by Dana, since 

 this volcano certainly consists of material which has been " pushed up- 

 ward " during the process of "mountain-making," and which evidently 

 formed a part of the exterior portion or crust of the earth. 



The only way in which we could avoid designating all eruptive rocks, 

 including those of which modern volcanoes are built up, as Archgean, 

 would be by taking issue in regard to exactly what is meant by that 

 frequently used term, the " crust of the earth." A discussion of this 

 question would, however, lead to no very satisfactory result ; for it is 

 probable that hardly any two geologists would agi-ee in their views, if 

 required to set forth exactly what they considered to be meant by the 

 phrase in question. 



It is true that Dana does not follow out his own definition to its logi- 

 cal end ; since he, for instance, speaks of the eruptive rocks of the 

 Connecticut Valley, not as Archaean, but as Mesozoic. Others, however, 

 seem to have taken his language as it would appear on the face of it to 

 have been intended that it should be taken, and have called eruptive 

 rocks of various ages Archaian, without any reference to the geological 

 tinie of their appearance in their present position, but solely because 

 they appear to have formed a pai't of the original " crust." 



That this method of grouping and nomenclature is entirely unphilo- 

 sophical, it seems hardly necessary for us to state. The first and most 

 important question in geological research is always, To what period does 

 the formation under investigation belong] If stratified, what geological 

 age is indicated by the fossils it contains ? If eruptive, at what time was 

 it erupted ? To class geological formations of different ages together is 

 extremely undesirable, and can only be admitted when the treatment of 

 the subject is a purely petrological one ; as if, for instance, we should 



