LINDGREN AND 

 BANSOME. 



] GENERAL GEOLOGY. 13 



along the northern, eastern, and southern borders of tut, area. It is 

 well exposed on Squaw Mountain and is the granite of the El Paso, 

 Elkton, Ajax, Portland, Independence, and Gold Coin mines. Of 

 these two kinds of granite the Cripple Creek variety is probably the 

 younger. The Cripple Creek gianite, the gneiss, and the schist 

 together form a wedge-shaped area projecting into the Pikes Peak 

 granite from the west. The center of volcanic disturbance practically 

 coincides with the point of this wedge. 



The present investigation indicates some necessary modifications of 

 the earlier report in the way- of stronger emphasis on the intimate 

 genetic relationship of the rocks. The "phonolite," "nepheline x - 

 syenite," "trachytic phonolite," "syenite-porphyry," and "andesites" 

 of Cross are all very closely related and have been found to be in most 

 cases connected by intermediate types. They are clearly all slightly 

 divergent eruptive facies of one general magma characterized chemi- 

 cally by containing from 9 to 15 per cent of potash and soda, the soda 

 being always somewhat higher than the potash, particularly when the 

 comparison is made by molecular ratios. None of the massive rocks 

 can properly be called andesite, and although it can not be affirmed 

 that andesitic fragments are entirely absent from the usually much 

 altered volcanic breccia, the term "andesitic breccia" does not seem 

 applicable to this formation as a whole. It would be more accurate to 

 describe it as a phonolitic breccia, although in places it consists chiefly 

 of particles of the older rocks through which the Tertiary eruptives 

 broke. 



None of the massive rocks erupted from the Cripple Creek volcanic 

 center and now preserved in the district show any evidence of having 

 been surface flows. They are for the most part intrusive porphyries, 

 ranging in texture, however, from the granular so-called nepheline- 

 syenite near the town of Independence to the nearly aphanitic phono- 

 lite of the smaller dikes and sheets. Most of them will come under 

 the designations phonolite, trachytic phonolite, trachydolerite, and 

 alkali-syenite. The extensive underground workings show that the 

 " nepheline-syenite " does not cut the "trachytic phonolite," but that 

 the two rocks represent textural and, to some extent, mineralogical 

 facies of the same mass, while the trachytic phonolite in turn may pass 

 into phonolite. The trachytic phonolite is in some instances cut by. 

 dikes of phonolite, showing, as Cross has already pointed out, that the 

 phonolitic intrusions were not all synchronous. 



While it is undoubtedly true that much of the breccia in the north- 

 eastern part of the volcanic area rests upon a very uneven surface of 

 granite, gneiss, and schist, the results of field work during the last 

 season, favored by deep workings not in existence when the district 

 was originally surveyed, have emphasized the fact that the breccia 

 lying southwest of a general northwest-southeast line drawn through 



