500 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



and of metalization, yet, from the position of the overlying strata, a prepon- 

 derance of evidence points to the belief that, whether eruptive or not, it is still 

 of Archaean origin ; hence its relations with the later stratified series are only 

 those of rigid underlying masses, and the local metamorphism observed in the 

 limestones near the granites is strictly mechanical, and not to be mistaken for 

 the caustic phenomena of a chemically energetic intrusion. It should be men- 

 tioned, however, that it possesses, both in its interior composition and in a 

 peculiar conoidal structure, (dose affinities with tlie nnmistakably eruptive 

 granites of the Sierra Nevada ; and it is quite possible that subsequent study 

 will determine the presence here of two distinct granites, the one having a 

 regular bedding and belonging to the stratified Archosan series, the other of 

 conoidal structure and eruptive origin. The main body extends about twelve 

 miles northeasterly, from the traphyte slopes of the Traverse Hills to the 

 head of the Little Cottonwood Caiion. Its greatest north-and-south expansion 

 is through Lone Peak, a line about eight miles long. South of the mouth of 

 Cottonwood Canon a narrow isolated patch of granite appears involved in. the 



Archa;an schists Passing up Cottonwood Canon, no sharp line of division 



between the structureless granite and the bedded gneissoid form is observable ; 

 but there appear gradually more and more planes having an easterly dip, until 

 finally they approach the regularity of gneiss bed-planes, and the minerals are 



seen to possess a vague general parallel arrangement The mineralogical 



dilferences through all these bodies of granite are indeed slight ; changes of 

 texture and arrangement produce a decidedly varying petrological effect, but in 

 general they are granites, containing — besides the normal orthoclase, quartz, 

 and biotite — plagioclase, hornblende, titanite, and apatite in high proportion ; 



all but the apatite being visible to the naked eye There are present in 



this neighborhood, then, two distinct families of rocks : first the Archaean, 

 consisting of schists and granite ; second, the vast, conformable post-^Archami 

 group of sediments. Wherever observed, the region of contact between the 

 two families displays no marked metamorphism on the part of the sedimen- 

 tary series, and within the Arch;ran series no such transitions as would lead to 

 the belief that the granite is only a more highly metamorphic form of the crys- 

 talline sedimentary series ; on the contrary, the contact is so clearly defined 

 and the rocks are mineralogiually so dissimilar, that it is very evident that the 

 granite is either an intrusive mass or else an original boss over which the 

 Archooan sedimentary materials were deposited. While the granite itself bears 

 a very close resemblance to the Californian eruptive granites, its relation to the 

 flexed Palaeozoic strata would indicate that they were bent around a solid body, 

 not that a plastic granite intruded into the bent Palaeozoics. The absence of 

 granite dikes penetrating the immense sedimentary series would strengthen the 

 belief that the granite antedated it. It is also noticeable that the dip and 

 strike of the Archoian schists west of the granite body are entirely discordant 

 with the overlying Cambrian series, the former striking nortlu^ast and dipping 

 northwest, the latter striking northwest and dipping southeast, this unconform- 

 ability being preserved up to the contact. Supposing the whole Archccan hody 



