266 GEOLOGY OF THE HIGH PLATEAUS. 



mass supported upon four corner-stones. Where lavas are disjointed into 

 large blocks of this kind it is not uncommon to find them, in the last stages 

 of decay, taking the aspect of a heap of gigantic bowlders. Granites and 

 massive sandstones sometimes exhibit the same behavior. The compan- 

 ions of these blocks have in this case probably been carried off by ice into 

 the gorges, and thus, instead of being erratics, they are the source from 

 which many erratics have probably emanated. 



In addition to the trachytic rocks of Fish Lake Plateau, many flows 

 of augitic andesite and dolerite are also found. These occur as intercala- 

 tions between the trachytes, and are very numerous; but as they lie in much 

 thinner sheets their aggregate mass is much less. The augitic andesites are 

 older than the dolerites, and are seen in greatest frequency at the lower 

 horizons. They vary considerably in character, some being hardly dis- 

 tinguishable from the augitic varieties of trachyte, and having' a grayish 

 color, while others merge into dolerites. Several varieties were found, which 

 were of a bright red color, and which might, upon hasty examination, have 

 been very deceptive. The iron contained in these varieties appears to be 

 largely in the form of peroxide, and both the magnetite and augite have been 

 altered, not by ordinary weathering, but by some metasomatic change which 

 I have not met with elsewhere. It does not appear to be identical alto- 

 gether with that alteration which reddens the scoria of basaltic cinder cones, 

 though the two changes may have much in common. In these varieties 

 the plagioclase crystals are well developed and retain their lively polariza- 

 tion, and are exquisitely striated. 



No particular portion of Fish Lake Plateau could be designated as a 

 focus of the very many eruptions which constitute its mass. Nothing like 

 a cone or crater is anywhere discernible, unless in some spot there may yet 

 remain the ruins of such a feature so nearly obliterated as to escape ordinary 

 or cursory observation. The several beds appear to lie in well stratified 

 sheets, somewhat irregular in form, occasionally highly so, but on the whole 

 decidedly like a series of coarse sedimentary strata in their general group- 

 ing. This, however, does not necessarily involve the inference that the 

 lavas came from a distant source or were not erupted from numerous fis- 

 sures and orifices in the vicinity and within the plateau mass itself. In 



