1917] 



Moody: Breccias of the Mariposa Formation 



399 



Perisphinctes colfaxi Gabb, found in the railroad cut one mile south 

 of Colfax, and Olcostephanus lindgrcni Hyatt, which occurs in rusty, 

 brown, sandy slate about one-half mile west of the first locality. These 

 ammonites point to an Upper Jurassic age. 



Since these fossils have been obtained from beds with which the 

 breccias are interstratified, it was thought well to obtain what evidence 

 they furnish as to the climatic conditions under which the deposits 

 formed. In answer to a request for such information, Professor J. 

 P. Smith replied : 7 ' ' The fossils of the Mariposa do not give much 

 indication as to climate except that they are all of northern kinship. 

 That may not mean cold, though it possibly means a cooler climate 

 than the Middle Jurassic." 



The occurrence of ammonites in the slate, however, indicates that 

 this portion of the series at least is marine, and any hypothesis explain- 

 ing the origin of the beds must take cognizance of this fact. 



CLASSIFICATION OF RELATED CONGLOMERATES 

 General Statement 

 In arriving at a correct understanding of the origin of the Mariposa 

 breccias it will be necessary to review in a few words the various con- 

 glomerate types now recognized and to institute comparisons between 

 them and the rock under discussion. 



Such a study was made ten years ago by G. R. Mansfield 8 in an 

 investigation of the stratigraphy of the Boston Basin, and his results 

 have proved of great value in the preparation of this paper. He 

 divided conglomerates into six types, classified on the basis of their 

 origin, as follows: (1) marine, (2) fluviatile, (3) estuarine, (±) 

 lacustrine, (5) glacial, (6) crush-conglomerate. Of these (4) and (6) 

 require no further consideration since the Mariposa breccias are inter- 

 stratified with marine beds, and since the heterogeneity of the pebbles 

 precludes the possibility of their formation by the brecciation of any 

 single rock. There then remain for consideration marine, fluviatile, 

 estuarine, and glacial conglomerates. To these must be added an 

 important class of clastic deposits recently described by Professor 

 A. C. Lawson as fanglomerate. The origin of the Colfax rocks is 



t Personal communication. 



s Mansfield, G. R., The origin and structure of the Roxbury conglomerate, 

 Bull. Mus. Comp. Zool. Harvard College, no. 49, Geol. ser., vol. 8, no. 4, 1906. 



9 The petrographic designation of alluvial fan formations, Univ. Calif. Publ. 

 Bull. Dept. Geology., vol. 7, no. 15, 1913. 



