ROOFING SLATE OF IGNEOUS ORIGIN 1 9 



age planes — has, of course, no bearing on the question of the 

 derivation of the green slates. It may prove, however, to be of 

 some importance in determining the probable cause of the origin 

 of slaty cleavage in this particular portion of the Mariposa slate 

 belt. 



Disregarding this omission of data as to the relation of con- 

 tact to cleavage plane, the fact remains that the band of green 

 slate is not everywhere conformable to the original bedding of 

 the Mariposa slate series. It is, therefore, highly improbable 

 that it was an originally interbedded member of that sedimen- 

 tary series; on structural grounds it is probable that it represents 

 a mass of igneous rock, injected as a dike into the Mariposa 

 series and, subsequently to its intrusion, -so highly sheared as to 

 have a very perfect slaty cleavage. This probability is increased 

 when the chemical composition of the rock is considered. 



Further confirmation of this hypothesis is afforded by an 

 examination of the cross-section of the dike, which proves that 

 it is not homogeneous in texture throughout, but that it varies 

 in bands closely parallel to the contact plane. Along its con- 

 tact with the normal black slate, the green slate is very fine- 

 grained for an inch or so. Bordering this is a zone several inches 

 in width, of coarser texture, and drab-green color, which is fol- 

 lowed in turn by the typical "green slate." These differences 

 in color and texture are sufficiently noticeable to be readily 

 distinguishable by the quarry men and slate splitters. . The tex- 

 tural differences are such, in fact, that the layer immediately 

 next to the contact is discarded as a "ribbon," since it works 

 unsatisfactorily. It must be recollected, however, that these 

 layers are parallel to the contact, not to the "ribbon" or original 

 bedding of the black slate. 



Igneous rocks of the vimiity. — About five hundred feet west of 

 the present quarry the western edge of the Mariposa slate is 

 reached, a body of igneous rock limiting it in that direction. 

 This rock is described in the "Placerville Folio" as diabase. A 

 linear outcrop of amphibolite, trending about parallel with the 

 cleavage of the slates, is shown on the "Areal Geology" sheet 

 of this folio, near Kelsey, and some distance east of the Eureka 



