STUDIES FOR STUDENTS 177 



floodings, as in the shallow lakes of arid basins and those of river 

 flood-plains and in lagoons cut off from sea except at highest tide 

 or greatest storms. Desiccation breccia may be only as thick as 

 the sun-cracked layer. Where the dried blocks of the mosaic or 

 pieces of their upturned edges are assembled by the waves, the 

 fragments may be irregularly piled in rubble and should show some 

 wear. The matrix differs little from the fragments, and the breccia 

 is endostratic. A special variety is playa breccia. The cracks 

 of the sun-baked clay of the dried-up lake bed may be filled with 

 desert sand, and this accumulates also beneath the curled-up edges 

 of the cakes. Desiccation breccias have been described from the 

 Algonkian of Idaho by Ransome and Calkins, 1 and designated as 

 "mud breccias." The angular or slightly rounded fragments are 

 of argillite and are imbedded in a somewhat coarser-grained and 

 more arenaceous matrix. Sun-cracks are found in direct connec- 

 tion, and the angular fragments are supposed to be broken off from 

 the edges of flakes of mud curled up by drying in the sun. 



Volcanic subaqueous breccia. — Volcanic breccia deposited under 

 water may be distinguished from that laid on land by the sediments 

 on which it rests and by the bedding of the tuff. Subaqueous 

 tuff breccias, as remarked by Leith, 2 are distinguished only with 

 very great difficulty from water-laid elastics resulting from the 

 erosion of volcanic rocks. 



Shoal breccia.- — In this class of submarine breccias, and in reef 

 and beach breccias as well, disruption and assemblage both are 

 caused by waves and tides. The normal action of these agents is 

 to round and sort the coarser stuff they handle and to deposit it in 

 well-defined conglomerates. It is only under exceptional condi- 

 tions that they can assemble beds of fragments so little worn as 

 to constitute a breccia. 



Shoal breccia is formed by the action of waves and tides on 

 shoals due to dias trophic movements or to general aggradation. 

 In reef breccia, on the other hand, there is proof that the shallows 

 permitting wave-pluck are due to local upbuilding of the sea-floor. 



1 F. L. Ransome and F. G. Calkins, "Geology and Ore Deposits of the Cdeur 

 d'Alene District, Idaho," U.S. Geol. Surv., Prof. Paper 62, p. 31. 



2 C. K. Leith, Structural Geology (New York, 1913), p. 66. 



