BLACK HILLS GEOLOGY. 257 



needles piled up after the manner of a snake fence, are to be 

 observed, and follow with great faithfulness the crystal boundary 

 of the section, but leave between that and themselves a clear 

 rim free from inclusions. The interstices between the quartzes 

 .are small but filled by a kaolinized material, which is interpen- 

 ■etrated by a confused maze of the same minute needles of segi- 

 rine. These can be best .distinguished with the high power. 

 They are not shredded as are the larger varieties, but are ter- 

 minated. From their extreme minuteness, it is impossible to 

 •observe any pleochroism or to determine their optical properties 

 ■other than to note the parallel extinction. The other constituent 

 •of the rock is plagioclase. It occurs in crystals which gener- 

 ally show complete crystal boundaries, but are slightly decom- 

 posed at the borders. Measurements on twin lamellae showed 

 a maximum extinction angle of about 19 degrees, which places 

 the feldspar among the oligoclase andesines. 



2. Phonolite Family. 



Of all the rocks developed within the district. studied, perhaps 

 none attain an extent and importance equal to that of the phono- 

 lites. The quartz porphyries are the nearest to them in abund- 

 ance and in the districts outside of the area mapped probably 

 .show a greater development than has been here observed. The 

 relative abundance of the two rock types has not been ade- 

 quately represented on the map, for the quartz-porphyries occur 

 so frequently in small dikes, sheets and irregular masses that it 

 has been possible to indicate exposures hardly equal to one-half 

 •of their actual number. The phonolites, on the other hand, 

 •occur in large' masses and dikes, which, although exceedingly 

 numerous, are almost invariably fresh and, being easily differen- 

 tiated from all other types of rock, may frequently be traced 

 for great distances. They intersect all the other eruptives with 

 which they come in contact. 



The phonolite family, as developed in the district mapped, 

 includes a series of rocks of so extremely varied a range in tex- 

 ture and appearance that it would be utterly out of the question 

 to attempt a separate description of each occurrence on the 



