1917] 



Moody: Breccias of the Mariposa Formation 



405 



the Boston Basin, and by Atwood 28 in the Eocene of southwestern 

 Colorado. The probability of a glacial origin for the Mariposa breccia 

 will be considered later. 



Fanglomerates 



This term was proposed by Professor Lawson 29 as a petrographic 

 designation for the coarser portions of alluvial fan formations. 

 Aridity and high relief are postulated in ascribing a fan origin to a 

 deposit. The constituents of a fanglomerate range from the coarsest 

 material down to fine sand. Blocks of extraordinary size are apt to 

 appear at the apex of a fan, and occasional ones, a foot or more in 

 diameter, are found far down the slope where the average size of the 

 material is less than one inch in maximum dimension. A gradation 

 in size of material from coarse to fine is, however, to be anticipated 

 as one progresses from the apex to the lower slopes of such a forma- 

 tion ; the normal sequence is fanglomerate, arkose, silt, the latter 

 representing playa, lake or river flood-plain detritus. In any given 

 point within an alluvial fan formation sorting action is very imper- 

 fect ; the sorting is chiefly that represented by the gradational change 

 from coarse to fine material in a lateral direction. The cement of a 

 fanglomerate cannot be fine grained; silt only appears near the playa 

 end of a fan. In general the matrix of such a deposit will be medium 

 to coarse sand. Of course infiltration may later cement the rock to 

 such an extent that it will fracture indifferently through, rather than 

 around, the included fragments. The masses included in a fan- 

 glomerate can never have traveled a great distance ; they are usually 

 of the same composition as the steep mountain slopes at the base of 

 which they lie. Occasional water-worn boulders are to be found 

 admixed with angular to subangular material which makes up the 

 greater portion of the rock. 



The argillaceous matrix of the Mariposa breccia is considered a 

 serious objection to an hypothesis of origin involving typical alluvial 

 fans of the desert type. Further, since marine beds are certainly inter- 

 stratified with the breccias, even though they themselves were not laid 

 down in the sea, the conditions for the formation of typical fan- 

 glomerates were certainly not realized. The probability of desert- 

 fans having played a part in the origin of the beds under consideration 

 is, then, held to be rather remote. The conception of alluvial deposits 



28 Atwood, W. A., U. S. Geol. Surv. Prof. Paper, no 95-B, 1915. 



29 Op. cit. 



