CONCENTRIC WEATHERING IN SEDIMENTARY ROCKS. 427 
The Society then adjourned for lunch until 2.80 o’clock pm. Upon 
reconvening, Vice-President Emerson occupied the chair. 
In the absence of the authors the two following papers were read by 
title: 
ON THE OCCURRENCE OF CORUNDUM IN NORTH HASTINGS, ONTARIO 
BY A. E. BARLOW 
ESTIMATES AND CAUSES OF THE SHORTENING OF THE OUTER PART OF THE 
EARTHS CRUST 
BY C. R. VAN HISE 
This paper by Professor Van Hise is not printed in the Bulletin, but 
another paper is printed in its place, entitled “ Metamorphism of Rocks 
and Rock Flowage,” which forms pages 269-328 of this volume. 
In the absence of the author the following paper was read by G. O. 
Smith : 
CONCENTRIC WEATHERING IN SEDIMENTARY ROCKS 
BY THOMAS C, HOPKINS 
Concentric weathering, while a common occurrence in many fine-grained, mas- 
sive, igneous rocks, is rather uncommon in the sedimentary rocks, except in the 
form of concretions, such as clay, clay-ironstone, and flint concretions. In such 
cases the weathering only intensifies or emphasizes the concentric structure that 
already existed in the rock, due to segregation of foreign material. 
Several conspicuous examples of concentric weathering in apparently hopioeene. 
ous sedimentary rocks were observed by the writer in his field-work this summer, 
- which he takes the liberty of presenting to the Society, thinking they may have 
as much interest for others as they had for him. The accompanying photographs 
will show the character of the weathered surface fairly well. Four of them are 
taken from exposures in the Ohio valley below Pittsburg, and one along the Cone- 
maugh river, in Indiana county. 
Vigures 1 and 2, plate 27, show the appearance of a bed of argillaceous shale after 
an exposure of about 25 or 30 years. The exposure isthe face ofa rock cut on the side 
of the wagon road on the bluff of the Ohio river opposite Beaver, in Beaver county. 
The shale belongs geologically but a few feet above the Lower Kittanning coal-bed. 
A glance at the photograph will show the presence of a. double concentricity, the 
larger or outer one starting from the joint-planes, rounding off the corners, giving 
a rounded concentric-like form to the whole mass between the joint-planes. This 
in no essential particulars differs from the exfoliation common in rocks, except the 
conspicuous action at the joints. Inside the grosser structure smaller concentric 
bodies are plainly visible on both sides of the joint-planes in figures 1 and 2, plate 27, 
These smaller concentric bodies have no perceptible difference in either color or 
composition from the surrounding mass in which they occur, nor do they, so far as 
observed, affect the lines of lamination in any way, nor are they affected by them. 
