BLACK HILLS GEOLOGY. 211 



tain the sediments have not been removed, so that the phono- 

 hte is not exposed, until we have approached much nearer to 

 the top of the steeper portion of the hill. It has been difficult 

 to obtain data as to the dip of the overlying shales, for these are 

 covered to the east and south by a residual talus of the white, 

 almost aphanitic rhyolite, so extensively developed about the 

 town of Englewood. Even where they have been uncovered by 

 the innumerable prospect holes that dot the region, they have 

 been of so fine and easily contorted a character as to furnish no 

 reliable information. It is probable, however, were a series of 

 observations available, as may at once be seen from the section, 

 that the low flat nature of the laccolite would render the estab- 

 lishment of a distinct quaquaversal a matter of no little diffi- 

 culty. 



Still, in view of the fact that the semicircular incision made 

 by Whitetail gulch has cut almost through the heart of the 

 mountain and exposed both its lower and upper contacts, little 

 better proof of its laccolitic character could be desired. 



The symmetry of the intrusion is marred on the northern side 

 by a large dike of an extremely coarse quartz porphyry. This, 

 together with numerous other northwest-southeast-striking 

 dikes, has formed an effectual barrier to the further extension 

 of the intrusion in this direction. The rock itself is a dove-col- 

 ored phonolite of the trachytoid type. As one ascends the hill 

 from the Union mine, the rock can be seen standing in large, 

 roughly hexagonal columns, which possess the characteristic 

 platy cleavage of phonolite in such perfect development as to al- 

 most resemble a sedimentary rock, The accompanying photo- 

 graph (Plate VIII.), which was taken about half way between 

 the summit of the mountain and the bottom of the gulch, will 

 illustrate this. The plates are readily separated one from an- 

 other, and seem to be due partially to strains developed in cool- 

 ing and partially to weathering. They give out the clear ring- 

 ing sound from which phonolite has derived its name, and seem 

 to be but little decomposed. 



Considered as a whole. Sugar Loaf hill may be regarded as 

 a rather more perfect specimen of laccolitic intrusion than we 



