1 8 EDWIN C. ECKEL 



THE "GREEN SLATES." 



Appearance and structural relatiojis. — Perhaps the most striking 

 feature of the quarry of the Eureka Slate Co., as seen from the 

 old ground surface eighty feet above the present floor of the 

 quarry, is a light green band, four feet or so in width, that 

 extends vertically from top to bottom of the quarry, and is par- 

 ticularly noticeable on the higher east wall. This band furnishes 

 the "green slate" of the quarry men. The contrast in color 

 between this green band and the intense black of the fresh sur- 

 face of the rest of the slate is very striking. 



Viewed from the old ground level, one cannot determine 

 whether the green band is parallel to the slaty cleavage or to 

 original bedding; which planes, as noted earlier in this paper, 

 commonly differ only by ten degrees or so. At first sight, there- 

 fore, the green band might reasonably be considered to be a 

 mere color variation, due either to original differences in compo- 

 sition of the beds from which the green and black slates were 

 derived, or to a later change in the color of certain beds; and 

 this view has apparently been accepted by former observers. 



Closer study, however, removes this easy explanation from the 

 list of possibilities. Even a casual examination of a green slate 

 quarried from this band, and comparison with a slate of the 

 normal black type, are sufficient to prove that the two slates are 

 different in more than color; while a closer examination of the 

 character and structural relations of the green band, when seen 

 from the quarry floor below, suffices not only to emphasize the 

 distinction between the green slate and the black, but to suggest 

 a somewhat novel origin for the former. 



Relations of contact plane to cleavage and bedding. — On going 

 down into the quarry and closely examining the relations of the 

 two slates, the contact between the green and black slates is 

 seen to be, not parallel to the "ribbon" of the black slate, which 

 indicates the plane of original bedding, but cutting the ribbon 

 at a small angle — not over ten degrees. It is not, however, 

 certain that the contact plane is exactly parallel to the plane of 

 slaty cleavage, which also cuts the bedding plane at a small 

 angle. This detail — the relation of the contact line to the cleav- 



