Hobbs—The Correlation of Fracture Systems. 23 
feet accord with those described by Gilbert as occurring in the 
clays of Lake Bonneville and controlling the local drainage di¬ 
rections. The data were secured by Mr. Israel C. Russell, then 
his assistant, of whom Gilbert 2 says: 
“He found that the details of drainage were controlled by a 
compound and extended system of joints. The principal series 
trend almost precisely" north and south and the subordinate 
series east and west. They are all vertical and straight and 
(within each series) closely parallel. They are readily traced 
from top to bottom of the walls of the lateral ravines and not in¬ 
frequently a wall exposes a broad flat sheet face caused by the 
removal of the clay from one side of the plane of jointing. Else¬ 
where the faces of the bluffs are buttressed by square pilasters or 
ornamented by outstanding rectangular columns, the forms of 
which have been determined by r the two systems of joints . . . 
“The point of especial interest is that these joints have been 
developed in post-glacial time within the series of strata not per¬ 
ceptibly indurated and which repose undisturbed in the place 
where they were deposited. The strata are nearly horizontal 
and their inclination of less than V 2 0 northward is presumably 
the pitch slope of the bottom upon which they were thrown 
down. ’ ’ 
Similar joints were observed by Russell 1 in the clays of Lake 
Lahontan. He says: 
“The marly clays forming the upper and lower members of 
the Lahontan series usually break into prismatic and cubical 
blocks on weathering,- the vertical faces of the blocks are deter¬ 
mined by joint planes and the horizontal by planes of lamina¬ 
tion. In many localities a more pronounced jointing occurs, 
forming two approximately" vertical systems that are nearly at 
right angles to each other. Judging from the number of in- 
1 Gilbert, G. K. Post-Glacial Joints. Am. Journ. Sci., 3rd Ser., Vol. 
23, 1882, pp. 25-27. See also same author, Lake Bonneville. Mono¬ 
graph I, U. S. Geo-1. Surv., 1890, pp. 211-213. 
2 Russell, I. C. Geological History of Lake Lahontan. Monograph 
XI, U. S. Geol. Surv., 1885, pp. 162-163. 
ilddings, Jos. P. A Fracture Valley System. Journal of Geology, 
Vol. 12, 1904, pp. 94-105. PI. 
