20 EDWIN C. ECKEL 



quarry. Other amphibolite dikes occur to the north and south 

 of Kelsey. 



Several other dikes, not hitherto mapped, and important in 

 their bearing on the problem of the origin of the green slates, 

 were noted during the recent work. These dikes outcrop between 

 the quarry and the diabase body west of it, as narrow linear areas, 

 trending N. 25 ° W,, or thereabout. They are best shown, however, 

 in a tunnel which is now being driven for drainage and working 

 purposes. This tunnel starts in the Kelsey ravine, which marks 

 the boundary between the diabase and the Mariposa slates ; and 

 runs eastward to the quarry, coming in at about twenty feet 

 below its present floor level. 



This tunnel intersects three dikes, in addition to cutting the 

 green band of the quarry. Of these dikes, two show material 

 which is fairly massive, while the third, the one nearest to the 

 green band, seems to have a decidedly slaty structure, though 

 not sufificiently so as to be utilized as a source of roofing slate. 

 From these observations it would seem that the intensity of the 

 shearing, which resulted in the present slaty cleavage, increased 

 as did the distance from the contact of the Mariposa slate and 

 the diorite. 



Microscopic investigation. — The results of microscopic investi- 

 gation were of no particular service in this connection, owing to 

 the fact that sufficient fresh material had not been secured from 

 the various dikes. 



A specimen from the dike farthest from the quarry — and 

 nearest to the great body of diabase — was examined by Dr. 

 Whitman Cross, who reported that it was a gabbro. 



Its main constituents are augite and plagioclase, with a rude parallel 

 arrangement of the minerals, suggesting that the specimen was derived 

 from the neighborhood of an original dike contact. There is a small amount 

 of brown amphibole, probably paramorphic after augite. While several 

 joints are visible cutting the specimen, no actual shearing zones or planes were 

 detected in the two sections prepared from this specimen. 



Several specimens from the body of the "green slate" band, 

 and also from the contact between the '*green slate" band and 

 the normal black clay-slates, were submitted to Dr. Cross for 

 examination. The results of this study were unfortunately uncon- 



