77 
Though there was similarity of conditions with the two 
Shales, there were also great differences. When the Lower 
Shales were in process of formation the general tendency of the 
sea bottom was to sink, with the Upper, it was to rise, and the 
Fauna brought in for the most part with the former, passed 
away with the latter. 
On the Silurian and Devonian land surfaces which remained 
during the formation of the Mountain Limestone, great changes 
had been going on. At the close of the Upper Carboniferous 
Shales we have evidence that the Flora exhibits more of the 
Carboniferous than the Devonian type. By the denundation 
of this land surface the Upper Shales were gradually covered 
by a close grained ferruginous grit, the Millstone grit, and it is 
_ in this that we find the species of plants of a Carboniferous type. 
In the Millstone grit there are a few seams of Coal that are 
worthless, and this fact, coupled with the general scantiness 
of Plant remains, indicates that the conditions were not 
favourable to their growth, or that they had been destroyed. 
This we think is characteristic of the Lower Carboniferous 
generally. Dr. Dawson states* that the “Flora of the St. 
Johns series of Devonian rocks excel in number of genera and 
species the Lower Carboniferous as it exists in British North 
America,” indicating deterioration or destruction. 
Now what is the composition of the Millstone grit, and what 
effect would its constituents have upon decomposing vegetation ? 
Dr. Sterry Honr statest+ “that the scantiness of Floral remains 
in some of the older rocks may be quite accounted for by the 
iron diffused in the soils and waters of those early times, which 
not only rendered possible the accumulation of such beds of 
ore, but oxidised and destroyed the organic matter, which in 
later ages appeared in coals, lignites, pyraschists and bitumers.” 
* On the Flora of the Devonian Period in N.E. America. Quart. Jour. Geo. Society, 
Vol. 18, page 304, 1862. 
t On the Origin of Metalliferous Deposits. Chemical and Geological Essays, 
page, 229. 
