WEATHEEING- INFLUENCED BY STEUCTUEE 233 



SO typical of the decomposition of massive rock is, as already 

 suggested, due wholly to external causes. W. P. Blake in 1855 

 called attention to this form of disintegration in the massive 

 sandstones near San Francisco (California) and pointed out 

 the true explanation.^ 



This sandstone is described as occurring in the form of layers 

 from a few inches to 6 and 8 feet in thickness, alternating with 

 beds of slate and shale. Down to a depth of 10 or 20 feet, or 

 to the limits of atmospheric action, all the beds have turned from 

 gray to rusty brown or drab. ''There are, however, parts of the 

 upper beds that have not yet been reached and changed by de- 

 composition ; these parts are found in the condition of spherical 

 or ellipsoidal masses, from which the weathered parts scale off 

 in successive crusts. These nuclei have the appearance of great 

 rounded boulders, and have accumulated in great numbers at 

 the base of the cliff." In this case the sandstone is composed 

 mainly of grains of quartz and a little feldspar cemented by 

 calcite, the disintegration being due mainly to the removal of 

 this cement by percolating water, while the change in color is 

 doubtless due to oxidizing pyrite or ferrous carbonate. 



The effect of percolating waters is not, however, always im- 

 mediately destructive. Through the presence of cementing ma- 

 terials in solution or by causing an oxidation of the iron car- 

 bonates or sulphides, a local induration may be induced along the 

 joint lines such as becomes conspicuous only through the weath- 

 ering away of the non-indurated portions. Eesultant forms may 

 be extremely regular or again irregular, according to the char- 

 acter of the lines along which percolation takes place, and that 

 of the rock itself. An interesting illustration of this form of 

 weathering is that given by Wyville Thompson^ as occurring 

 in limestones on the islands of Bermuda. 



**This dissolving and hardening process," he writes, ''takes 

 place irregularly, the water apparently following certain courses 

 in its percolations, which it keeps open, and the walls of which 

 it hardens; and in consequence of this, the rock weathers most 

 unequally, leaving extraordinary rugged fissures and pinnacles, 

 and piled up boulders, the cores of masses which have been 



J Expl. and Survey for a Eailroad from the Mississippi to the Pacific 

 Ocean; Eeport on the Geology of the Eoute, near the 32d Parallel, by W. 

 P. Blake. 



2 See The Atlantic, Yol. I. Also Bull. 25, IT. S. National Museum. 



