STUDIES FOR STUDENTS 191 



Expansion breccia.- — Fragmentation may be caused by increase 

 of bulk of the brecciating rock or of associated layers which trans- 

 mit the pressure to it. Expansion may be caused by recrystal- 

 lization or by hydration. Expansion breccias graduate into folded 

 structures, but the folds show quaquaversal deformation, the 

 enter olithic structure of Grabau, 1 a term translating the "Gekrose" 

 of Koken, and this intestinal coiling may serve to distinguish them 

 and the associated breccias from folds and breccias due to lateral 

 pressure. 



Founder breccia. — Where beds of soluble rocks have been in 

 part or whole removed by the chemical action of ground-water, 

 founder breccias of the superincumbent beds are produced on a 

 scale commensurate with the extent of the ablation. All ter- 

 ranes between the dissolving foundation and the surface of the 

 ground or some competent superior stratum are affected by the 

 process. Characteristic features are abnormal dips, sag folds 

 without parallelism of axes, areas of crushed rock alternating with 

 horsts where the strata are undisturbed. The matrix may be of 

 crushed material of the same terrane as the fragments. In this 

 case it is likely to be small in amount and insufficient to cement the 

 breccia firmly, for the attrition of the fragments in founder is 

 probably much less than in tectonic brecciation. The matrix may 

 be a later chemical deposit. 



The fragments show only the small wear due to mutual attrition. 

 Like other endolithic breccias, a founder breccia can carry no 

 water-worn pebbles. It is conceived that founder breccias of thick 

 extensive beds include larger blocks than any other type of breccia 

 excepting that due to landslide. Certainly they may be far too 

 great for detachment by waves or by mechanical weathering and 

 for fragmentation under lateral pressure. Horsts may be difficult 

 to discriminate from the undisturbed areas of tectonic breccias. 



Thin-shale founder breccias have been described by Ransome 2 

 in the blankets of several mines in the Rico district, Colorado. 

 The blanket of the Enterprise mine, for example, is an uncon- 

 solidated breccia of shale from 2 to 20 feet thick, resting on a thin 



1 Principles of Stratigraphy, p. 757. 



2 F. L. Ransome, "Ore Deposits of the Rico Mountains, Colorado," U.S. Geol. 

 Surv., 22d Ann. Rept., Part II, p. 273 f. 



