1917] Moody: Breccias of the Mariposa Formation 



413 



IGNEOUS ROCKS IN THE MARIPOSA 

 Dikes and masses of diabase and serpentine intrude the Mariposa 

 formation at various localities, while a gabbro stock invades its 

 northern limits. Their occurrence may be dismissed with the statement 

 that little or no contact metamorphism has been induced in the sedi- 

 ments by their intrusion. 



RESUME 



The Mariposa is a comfortable, gradational series of breccias, 

 sandstones and slates, intruded by a gabbro mass in its northern 

 limits. Structurally the beds have suffered isoclinal folding which, 

 toward the north, becomes more open. Slaty cleavage parallels the 

 bedding in the sandstone but frequently cuts the stratification planes 

 of the slate. 



All gradations occur in the sediments from fine mud up to masses 

 a yard or more across. The coarser material is dominantly angular 

 to subangular, although well rounded pebbles, chiefly quartz, do occur ; 

 the matrix is arenaceous to silty. The recognizable fragments could 

 all have been derived from the associated Calaveras rocks. 



Suggestions of glacial action or at least of the transporting agency 

 of ground-ice or of icebergs appear in the sediments, but high grade 

 streams consequent upon folding or faulting in a northwest-southeast 

 direction, frequently rejuvenated by recurrent movements, may have 

 provided the coarse material which makes up the breccia ; and low 

 grade streams, flowing longitudinally with respect to the Mariposa 

 troughs, may have transported the fine detritus and more rounded 

 pebbles, now forming a large part of the deposit. 



The chief difficulties to be met in the way of a glacial hypothesis 

 of origin are (a) lack of positive glacial evidence; (6) the large 

 number of alternations of slate and breccia. Any normal process of 

 alluviation, however, fails to explain the occurrence of sporadic pebbles 

 and boulders in a silty matrix. 



