PROWERSOSE FROM TWO BUTTES, COLORADO 169 



The microscope shows the presence of serpentine and chlorite 

 occupying areas which probably represent original crystals of olivine. 

 Some are large, corresponding to those seen in the hand specimen, 

 while others are very small. The alteration is so extreme, how- 

 ever, that neither by outhne nor by indications of the course of 

 alteration can one positively determine this matter. 



Magnetite occurs in very numerous particles only o.oi™™ to 

 0.02"^™ in diameter, and apatite is chiefly developed in minute 

 needles, with here and there a large stout phenocryst, comparable 

 to the augite in size. 



The obscure character of the salic constituents led to a careful 

 search for leucite and nephehte, without evidence of either being 

 found. The 5.81 per cent, of the rock soluble in dilute nitric acid 

 contains no appreciable soda, and hence nephelite and zeohtes 

 derived from it are excluded. 



On the basis of the above description, this rock may be called, 

 in the prevailing nomenclature, a syenitic lamprophyre allied to 

 minette. 



The rock contains some small angular and sharply defined inclu- 

 sions of white color and fine granular texture, which are seen in 

 thin section to consist mainly of orthoclase and very obscure micro- 

 perthite, with possibly a small amount of some highly sodic plagio- 

 clase. These minerals are developed in anhedral grains, and resemble 

 the orthoclase of the rock only in their dustUke interpositions. A 

 few flakes of biotite, prisms of augite, and grains of magnetite larger 

 than those of the rock are scattered through this feldspathic mass. 

 It seems to represent material genetically related to the host, but 

 derived from some deep-seated source. There is no suggestion 

 of fusion or assimilation of the included particles. 



Classification by the quantitative system. — The norm of the Two 

 Buttes lamprophyre is given in Column I of the accompanying 

 table. For comparison the norms of the other rocks of which 

 analyses have been given are appended. The figures for IV are 

 taken from Bastin's paper; for V and VI, from Washington's 

 tables; and for VII, from Pirsson's bulletin on the Highwood 

 Mountains. 



The position of the Two Buttes rock in the quantitative system 



