1917] 



Moody: 



Breccias of the Mariposa Formation 



401 



angular material predominates in every exposure of the Mariposa; in 

 some, no rounded material is visible; (2) the pebbles are unsorted, 

 distinct bedding is often lacking; (3) the matrix is largely silty. 



Pluviatile 



Conglomerates referable to stream action usually have a matrix of 

 sand mixed with fine and coarse material which is, in general, not well 

 sorted and may be cross-bedded ; the individual grains vary from 

 angular to partly rounded. The coarser material ordinarily consists 

 of locally derived masses which may range up to several tons in weight ; 

 the boulders vary from well-rounded to subangular. Frequent alter- 

 nations of coarse and fine beds, both commonly showing current mark- 

 ings, oblique lamination and numerous irregularities in thickness and 

 composition characterize such deposits. The thickness of flood-plain 

 conglomerates may be very great. 



Instances of conglomerates of this type are to be found all over 

 the world. The Siwalik Group, a Tertiary deposit of India, with a 

 maximum thickness of 14,000 feet is regarded by Geikie 14 as of fluviatile 

 origin. Fluviatile deposits occur in the upper Old Red Sandstone of 

 Great Britain, 15 in the High Plateaus of Utah, described by Dutton, 10 

 where angular material predominates and in the Great Valley of 

 California as discussed by Ransome. 17 The auriferous gravels of the 

 Sierra Nevada form one of the most typical of fluviatile deposits yet 

 described. 



The predominance of rounded and subangular pebbles in typical 

 fluviatile conglomerate is not realized in the Mariposa beds. As will 

 appear in the sequel, however, it is believed that streams of high grade 

 may have been instrumental in transporting the material which now 

 makes up the formation. 



The deposits termed "estuarine" 18 by Mansfield are not essentially 

 distinct from those of fluviatile origin ; as he uses the term it is appar- 

 ently intended to be restricted to the deposits accumulated in the delta 

 portion of streams. This class of deposits is summarized by Bailey 

 Willis 10 in the following statement : 



The unassorted mingling of sandy and clayey particles is the result of 

 rapid deposition at the mouths of muddy streams in opposition to waves that 

 are too weak to sort and distribute the volume of sediment. This is a condition 



14 Geikie, A., Textbook of Geology, ed. 3, p. 1021. 

 is Op. cit., ed. 3, p. 802. 



is Report on the geology of the high plateaus of Utah, 1880. 



it Ransome, F. L., Univ. Calif. Publ. Bull. Dept. Geol., vol. 1, no. 14. 



is Loc. cit., pp. 121-122. 



19 Willis, Bailey, Maryland Geol. Surv., vol. 4, p. 63, 1902. 



