54 



PROCEEDINGS OF WASHINGTON MEETING. 



structure on either side has an average thickness of 3 inches. The blocks into 

 which the wall is broken are short in proportion to thickness, and many might be 

 called cuboidal. The stone, too, is harder than in number 1. The direction of the 

 dike is N. 70° E. 



If the lines of direction of the two dikes be continued, they cross a little nearer 

 to number 1 than number 2, and at an angle (as seen from the above figures) of 22 

 degrees. Running these lines without surveying instruments, this angle was 

 obtained as 25 degrees. Considering the distance apart and the smallness of this 

 angle, it might be possible that the two exposures are really parts of one curved 

 dike. 



I 



Figure 21 — Dike number 2. 

 Showing characteristic "manvaise terre" erosion. 



Professor F. R. Carpenter, of Rapid City, South Dakota, a fellow of this Society, 

 has had the following analysis made by Mr. Barnett, one of his assistants. The 

 analysis is of a piece from dike number 2: 



Si0 2 77.S4 



A1 2 3 13.09 



Fe 2 3 1.26 



CaO 3.41 



MgO tr. 



H 2 3.20 



98.80 



