108 ADDITIONAL OBSEEVATIONS ON KH^TIC BEDS. 
sulphate formed, reducing this again to iron sulphide, possibly 
in the form of marcasite, known to occur in the Green Marls; 
I am aware that this suggestion implies, aybrf^oH', that 
the whole of the Grreen Marls were produced by the decom- 
position of carbonaceous mud through the agency of pyrites; 
but in the present uncertain state of knowledge as to their 
conditions of deposition, this may not be deemed impossible. 
If they are so produced, the curious shales I have described 
lose no interest as true passage beds, and show that the 
Rhsetic Beds graduate as insensibly into the underlying 
Trias as into the overlying Lias.^ 
If the brown clay (iii.) is the result of decomposition — ^^and 
I have seen just such a clay produced by weathering of 
pyritous Black Shales — the sandstone (ii.) would be the actual 
base of the Black Shales. It occurs in narrow bands, at 
most half an inch in thickness, with clay partings, and is a 
light-coloured, fine-grained sandstone, somewhat micaceous,^ 
smooth on one surface ; the other surface is irregular, and 
shows larger, well-rounded grains of clear and white quartz, 
and of dark chert, with some glauconite. 
In the clay adherent to this surface occur distinct, though 
scanty and ill-preserved, fish remains, but these would not 
in any way justify the denotation of this sandstone as a 
"Bone-Bed." 
^ A distinction might be drawn between beds that are physically 
transitional, and those that are transitional in respect of their 
organic remains. The Ehsetic fauna is at least as distinctive as 
that of many greater formations. 
