Vol. 60.] SECTIONS IX THE BRISTOL DISTRICT. 183 



laminated black shales near the bottom of the series, well deserving 

 the name of ' Paper-Shales ' when dry. This zone is very barren. 



Mr. Parsons has observed, at the base of the Black Shales at Red- 

 land, a very curious type of passage-beds between these shales and 

 the underlvins: beds, which are often vellowish-brown shales or 

 clays, sometimes typically-green laminated marl. Instead of a bed 

 of well-marked Black Shale resting upon one of brown or green marl, 

 the shales of the two series dovetail one into the other, and in one 

 and the same bed pieces in some parts black, in others green, may be 

 found. Moreover, it is common to find a horizon chiefly of Black 

 Shale overlain by one chiefly of green, or red, or yellow shale or marl. 

 In fact, at the junction every variety of admixture may be noted. 

 Mr. Parsons explains it by assuming that the whole was originally 

 black, and has been bleached by oxidation through the agency of 

 pyrites. He is even prepared to assume that the whole of the Green 

 Marls were so produced (47). 



Concerning the truth of his observation there is no doubt. But I 

 think that a much more probable explanation is that the Black Shales 

 are simply the brown or green clays with organic matter added. 

 Just when the Rhnstic sea entered and brought this animal life, it 

 is not at all surprising that it should be patchy in distribution at 

 first. Then the interlocking would be readily explained. This is 

 all but proved by a somewhat parallel case. In my description of 

 the shale just above the Cotham Marble at Redland, numbered 0, 

 I have mentioned that it is usually yellow, and is barren. But in 

 one place it becomes fossiliferous, and has yielded Modicila minima, 

 Moaotis deeussata, etc. (see p. 17-1). And here it is no longer yellow, 

 but black: of exactly the same appearance, in fact, as the Avicula- 

 contoria shales. 



The Black Shales include several inconstant bands of siliceous or 

 pyritous grit, often marked with the obscure fossil that was for- 

 merly called ' Pvllastra? They are somewhat pyritous all through. 

 Hippie-marks (27) and sun- cracks are recorded (31). Occasionally, 

 seams rich in teeth and scales and coprolites occur, which have 

 been diguified with the name of Bone-Beds, as at Emborough in 

 the Mendips (48), and at Garden Cliff (2). At Cotham, in the 

 section that I have described, elytra of beetles probably occur. 



What conclusions can we draw from the foregoing observations 

 as to the physical conditions of deposition of these beds? After 

 the Bone-Bed storms, the depth of the Avater evidently increased, 

 partly due to rains, and partly to depression of the land. The 

 salinity had now been corrected, and the open communication with 

 the sea kept up the relative freshness. In fact, the Rhaetic Period 

 in England seems to have been a good deal more rainy than the 

 Triassic, and the waters ultimatelv became brackish. During the 

 Black-Shale time-interval, however, they were probably of ordinary 

 oceanic salinity. Although deeper than before, the Black-Shale 

 sea was still shallow, and occasionally patches were left dry to 

 sun-crack. Rippling and the occurrence of insects also indicate 

 fairlv-sh allow water. 



