256 GEOLOGY OF THE EUREKA DISTRICT. 



changes in the mineral and chemical composition of this magma are by no 

 means as easy to follow through the different flows as in the sequence of 

 outbursts of the acidic products. Nevertheless, investigation shows as com- 

 plete a range in composition of the erupted material, even where it is 

 impossible to determine the relative age of the flows accompanying such 

 changes. In some instances in the more crystalline acidic varieties it has 

 been pointed out that both hornblende and mica occur as porphyritic secre- 

 tions, although as accessory constituents, hypersthene being the predom- 

 inant mineral. In lavas slightly more basic the former minerals are want- 

 ing; hypersthene still plays the part of the prevailing ferro-magnesian 

 silicate, accompanied by relatively small amounts of augite, while in rocks 

 still more basic augite is recognized as the predominant pyroxenic mineral, 

 accompanied by an increasing development of magnetite. By insensible 

 gradations a series of hand specimens and rock sections show that so far as 

 mineral constituents are concerned the pyroxene-andesites pass over into 

 basalts. While the rock masses of both lavas may be readily distinguished 

 in the field by marked differences in physical aspect, it is by no means 

 easy on a superficial examination to refer correctly from hand specimens 

 certain varieties which approach each other in structure and composition. 

 Mineralogically no sharp distinction can be drawn between intermediate 

 varieties, but careful investigation of the Eureka rocks brings out certain 

 differences which not only hold good for this region, but probably for other 

 areas in- the Great Basin. While observation, as already mentioned, offers 

 abundant evidence as to the position of the pyroxene-andesite to the basalts 

 and divides these closely connected rocks upon geological grounds, based 

 upon their relative age, the microscope in a marked manner corroborates 

 the distinctions made in the field. Mr. Iddings, who has submitted a large 

 number of thin sections of both pyroxene-andesite and basalt to micro- 

 scopical investigation, is able to substantiate by structural peculiarities of 

 the groundmass the geological divisions observed. He finds that all those 

 rocks which have been classed as pyroxene-andesite possess their own micro- 

 structure, characterized by the felt-like structure of the groundmass which 

 has been so frequently noticed elsewhere. The typical basalts present in 



