STRUCTURAL GEOLOGY, 



509 



may be determined may be few and far between, but whea they are 

 sufficiently near one another, the structure of the rock, as shown in Fig. 

 405, may be worked out, even though the surface be flat. 



Much the larger portion of the earth's surface is occupied by beds 

 that retain nearly their original horizontal attitude ; but in mountain- 

 ous regions the beds have usually suffered bending, folding, crump- 





Fig. 410. — Plicated layers of thin-bedded chert in limestone, etched by erosion. Lower 

 Cambrian (?), two miles southwest of Big Pine, Inyo Co., Cal. (Walcott, U. S. 

 Geol. Surv.) 



ling, and crushing, in various degrees, in the course of the deformations 

 that gave rise to the mountains. Distortion is on the whole most intense 

 and characteristic in the most ancient rocks known, the Archean, in 

 which a distorted condition is nearly universal, so far as observation 

 goes. Distortion is assigned chiefly to lateral thrust arising from the 

 shrinkage of the earth, as explained in the chapter on Earth Movements. 



