400 University of California Publications in Geology [Vol. 10 



certainly comprised in the processes which have given rise to the above 

 mentioned types, but the fact must not be lost sight of that two or 

 more processes may have operated to produce the breccias. 



Marine 10 



Typical marine conglomerates have clean, well-marked and sorted 

 sand as a matrix. When in quantity the sand may be more or less 

 cross-bedded ; the grains vary from angular to rounded. The com- 

 ponent pebbles of the rock are usually of material of local origin ; 

 they are uniform in size and are ordinarily well rounded. The strati- 

 fication of marine conglomerates is usually well marked and cross- 

 bedding may be more or less perfectly developed. In the normal cycle 

 of sedimentation, following an encroaching sea, finer material will 

 overlie the coarse and may locally interdigitate with it. Local un- 

 conformities and irregularities may be expected along the dip but great 

 persistence and uniformity is common along the strike. In general, 

 true marine conglomerates are not thick; Barrell 11 states that their 

 thickness is usually less than one hundred feet. 



Mansfield has cited the Cretaceous of Texas as a typical example 

 of a series of marine deposits. The Trinity sands, described by Hill, 12 

 as the basal formation, have as their lowest member a fine pebble 

 conglomerate, consisting of small masses of the adjacent pre-existing 

 rocks in a cement of ferruginous yellow and red gritty sand. Most 

 of the coarser material is well rounded and sorted, though in some 

 localities it is predominantly subangular with only the more resistant 

 quartz, which has travelled a long distance, well rounded. The basal 

 conglomerate grades upwards into sands, grits, and silts; it is seldom 

 over two hundred feet thick. Both massive and well-bedded deposits 

 occur in the formation ; sands and clays are interstratified with the 

 conglomerate in places, while elsewhere the beds are dominantly coarse. 



Other typical marine conglomerates, deposited in an encroaching 

 sea, are abundant in the stratigraphy of the western part of the United 

 States. Among them might be mentioned the Scanlon and Barnes 

 arkosic conglomerates described by Eansome 13 from the Globe district. 



The following points of difference may be pointed out between the 

 Mariposa breccias and typical marine deposits of that type: (1) 



10 Mansfield, loc. cit., pp. 107-150. 



11 Barrell, J., Abstract: Bull. Geol. Soc. Amer., vol. 20, p. 620, 1909. 



12 Hill, B. T., Twenty-first Annual Beport, U. S. Geol. Surv., pt. 7, 1901. 



13 Eansome, F. L., U. S. Geol. Surv. Frof. Faper, no. 12, pp. 30-31, 1903. 



