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Show 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 companions 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 intercalations 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 distinguishable 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 altogether 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 polarization, 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, b^t on the whole decidedly like a series of coarse sedimentary strata in their general grouping. This, however, does not necessarily involve the inference that the lavas came from a distant source or were not erupted from numerous fissures and orifices in the vicinity and within the plateau mass itself. In |