LITHOGENESIS OF EXOGENETIC ROCKS 527 



been described from the Cambric of North America, and from 

 other deposits. (See further, Chapter XXII.) 



AUTOCLASTIC ROCKS. 



We may define an autoclastic rock as made of the fragmental 

 material resulting from the movement of one rock mass over an- 

 other, both contributing from their substance to the clastic material, 

 though, owing to different degrees of hardness, that furnished by 

 one of the masses may be most prominent, while that contrilnited 

 by the other may be almost if not wholly destroyed during the 

 movements. 



Classification of Autoclastic Rocks. According to the type 

 of movements in the formation of an autoclastic rock we may 

 classify the products as follows : 



1. Exogenous, or those due to external forces, including: 



a. Those due to compressive strains, i. e., thrust-fault brec- 



cias. 



b. Those due to gravitational strains, comprising: 



(i) Breccias, etc., formed by ordinary gravity or normal 

 faulting subsequent to the formation of the strata, a 

 secondary structure, and 



(2) Breccias and brecciated structures formed by slip- 

 ping or gliding of material during the process of ac- 

 cumulation under the influence of gravity, and re- 

 sulting in the production of intraformational brec- 

 cias, or, when the brecciation is incomplete, contorted 

 stratification. Glacial deposits are best placed here. 



2. Endogenous or endolithic structures, or those due to in- 



ternal forces, including: 



c. Those due to movements resulting from solution and re- 



crystallization, as in ice, salt, etc. 



d. Those due to increase or decrease in volume from hydra- 



tion, carbonation, oxidation, etc., or the reverse, as 

 in change of anhydrite to gypsum, or the removal of 

 certain elements, as in dolomitization of limestones. 

 The distortion of salt layers, of gypsum beds, and 

 limestones producing the contorted structure which, 

 from its resemblance to the coiled intestine, has been 

 called enterolithic structure (German, Gekrose), may 

 be cited as illustrations. 



The more important types may be described in detail. 



