160 THE PRINCIPLES INVOLVED IN ROCK-WEATHERING 



States Geological Survey, has photographed quite similar phe- 

 nomena in the high Sierras of California. (See Fig. 1, PL 13.) 



Shaler states^ that rock surfaces in the eastern United States 

 may be subjected to temperature varying from 150"* F. at 

 midday in summer to 0° and below in winter. This change of 

 150° in a sheet of granite 100 feet in diameter would produce a 

 lateral expansion of about one inch of surface. That this ex- 

 pansion must tend to lessen the cohesion and tear the upper 

 from the deeper lying layers, is self-evident. As exemplifying 

 this, Professor Shaler states that there are on Cape Ann (Massa- 

 chusetts) hundreds of acres of bare rock surface completely 

 covered with blocks of stone, which have been separated from 

 the mass beneath by just this process.^ 



The size of such flakes may vary from those of microscopic 

 proportions to masses of several tons' weight. The higher 

 slopes of Lone Mountain, east of the Madison, in Montana, are 

 covered above timber line with thousands upon thousands of loose 

 flakes of all sizes up to ten or more feet in diameter. Such, here, 

 as in general, are characterized by a roughly lenticular outline 

 in cross-section, possessing a large superficial area in proportion 

 to their thickness, and are further distinguished from boulders 

 of decomposition by the entire freshness of their materials even 

 to the very surface. In close-grained, black andesitie and basaltic 

 rocks the chip or flake not infrequently shows a beautiful concave 

 and convex form and is greatly elongated in proportion to its 

 breadth, resembling the long and slender chips of obsidian or 

 flint found on the sites of aboriginal workshops. The surface 

 left by the springing off of the flakes is fluted as though the work 

 were done with a carpenter's gouge. 



In regions of great extremes of daily temperature the rup- 



^ Proc. Boston Soc. of Nat. History, XII, 1869, p. 292. 



2 The rifting action of heat upon granitic masses is said to have been 

 made a matter of quarry utility in India. It is stated (Nature, January 17, 

 1895) that a wood fire built upon the surface of the granite ledge and 

 pushed slowly forward causes the stone to rift out in sheets six inches or 

 so in thickness, and of almost any desired superficial area. Slabs 60 X 40 

 feet in area, varying not more than half an inch from a uniform thickness 

 throughout, have been thus obtained. In one instance mentioned, the sur- 

 face passed over by the line of fire was 460 feet, setting free an area of 

 stone of 740 square feet of an average thickness of five inches. This stone 

 is undoubtedly one of remarkably easy rift, but the case will, nevertheless, 

 serve our present purposes of illustration. 



