BLACK HILLS GEOLOGY. 253 



tion with the main mass or radially like the spicules of a radio- 

 larian. 



The fine needle-like aegirines seen in the groundmass of the 

 Lost Camp creek and Elk mountain types, are almost if not 

 completely absent. The feldspar phenocrysts are but little de- 

 composed, and then the alteration seems to be only a slight 

 kaolinization, although secondary quartz was observed in one 

 or two of the more decomposed specimens. 



The feldspar of the groundmass is partly automorphic, and 

 partly contemporaneous with the quartz in the order of crystal- 

 lization. It can be easily distinguished from the quartz by its 

 cleavage and kaolinized appearance. . A few small automorphic 

 crystals of a very acid plagioclase are present. 



The chief difference between this rock and that of the two 

 types previously described is the greater predominance of the 

 sanidine and segirine-augite phenocrysts and the almost com- 

 plete absence of the network of minute aegirine needles from 

 the groundmass, the smaller aegirines being confined chiefly to 

 the hair-like enlargements of the larger crystals. Microcline 

 also is absent. The quartz shows no unusual characters. A 

 silica determination made by Caswell shows Si02 71.13 per 

 cent. 



4. Annie Creek Type. 



From the Terry peak type we can pass to that from the head 

 of Annie creek by a very slight gradation. The rock described 

 was taken from the conspicuous dike-like mass, northeast of 

 Foley peak. In the Terry peak type, as in all of the others 

 yet described, the quartz is wholly confined to the groundmass. 

 In this rock, however, we find large and quite numerous pheno- 

 crysts of quartz, which range from -Jg- inch to as much as 

 3/S inch, and in rare instances reach i^ inch in diameter. The 

 average is about }( inch. They are but little rounded by ab- 

 sorption and exhibit the usual bi-pyramidal character. In 

 many instances a marked zonary banding may be observed, 

 even in the hand specimens. , 



When examined under the microscope this banding is seen to 



