L. V. Rirsson — Phonolilic Rocks from Black Hills. 341 



Art. XXX VI. — On some Phonolitic Rocks from the Black 

 Hills; by L. V. Pirsson. 



For our knowledge of the geology of the Black Hills and 

 of the occurrence and distribution of the igneous rocks of the 

 region we are chiefly indebted to Newton.* Crosbyf has also 

 shown that the greater part of the younger eruptives are to be 

 considered as occurring in the form of laccolites. Our infor- 

 mation in regard to the petrographical character of these rocks 

 is due to Caswell,:}: who studied the suite of specimens gath- 

 ered by Newton. This was one of the earlier contributions to 

 American petrology, and when we consider the amount of 

 knowledge available on the subject at that time, it was an 

 excellent and careful piece of work. 



Reading between the lines of Caswell's report however and 

 possessed of the facts which the great advance in petrograph- 

 ical knowledge during the past fifteen years has furnished we 

 can see that many of the rocks studied by Caswell were of 

 great interest, unusual in character and that they possessed re- 

 lations not then brought out. 



This was shown by an examination of several specimens 

 from the region which have come into the writer's hands from 

 various sources to be mentioned later and it was therefore de- 

 cided to study them more in detail with the results given in 

 the present article. 



Phonolite. 



This rock is definitely mentioned by Caswell, as occurring 

 at Black Butte and he gives a figure and description of it.§ 

 From the facts stated by him it would seem to be present also 

 in other localities. Thus the rock from Bear Lodge, or as it is 

 now commonly called, the " Devil's Tower," an isolated butte 

 near the Little Missouri Buttes on the northwest border of the 

 region and described by Newton|j as having a remarkable 

 columnar structure is said to gelatinize readily in acid. It is 

 described as sanidine trachyte. 



A specimen of this rock has come into possession of the 

 writer through the kindness of Mr. Geo. Leavenworth of JSIew 

 Haven, who visited the locality in 1889. Both in macro- 

 scopic appearance and in thin section it agrees quite well with 

 the description given by Caswell but differs in some important 

 characters. It may also be reasonably inferred that the rock 



* Geol. of Black Hills of Dakota, U. S. Geog. and Geol. Surv. Rocky Mts., 

 Washington, 1880. 



f Proc. Bost. Soc. Nat. Hist., vol. xxiii, p. 511, 1888. 



% Geol. Black Hills, as above, Chap, vn, p. 471, Micro. Petrog. 



§ Op. cit, p. 503. Plate I, fig. 3. 



[Op. cit., p. 201. 



