162 EDWIN GILPIN ON LIMESTONES 
faults and undulations. The Grant limestones (No. 6.) are apparently, about the middle 
of this formation, and may be paralleled with the Forbes, McLellan and Robertson 
limestones (7, 8, 9). 
On the West Branch, a short distance above the forks, is a limited exposure of gypsum, 
which is associated with gray arenaceous limestone, and a series of thick bedded sand- 
stones, holding Calcopyrite casts of plant remains. Similar limestones are met on the 
extension of the strike of these measures to the East Branch, and a bed of limestone (No. 
10) several feet thick, composed of minute fragments of fossils, which give a rough pumice- 
like surface on weathering. This is, probably, the limestone referred to in “Acadian 
Geology” (p. 318), as showing in slices under the microscope, that it is made up of small 
fragments of shells, with entire specimens of very minute species. 
Some of the limestones are well defined and persistent. At other points they are 
quite local. It sometimes appears as if there had been a local accumulation of calcareous 
matter (of shells or of a coral growth) which rapidly thinned from a central point, until 
lost in argillaceous or arenaceous matter. ; 
The gypsum at the Forks may be considered as marking an horizon very near the 
summit of the Marine Limestone formation. It is difficult to arrive at any exact estimate 
of the total thickness of this formation in the district under consideration, starting from 
the basal limestone of McPhee’s, and ending at the Forks. The longest continuous section 
that I have been able to measure, did not exceed 1,040 feet, but from all available data, the 
total thickness may be estimated at about 2,750 feet. 
Below the forks of the river, measures referred by the officers of the Geological Survey 
to the Millstone Grit, are met as far as the base of the Productive Coal Formation, a short 
distance north of McKay’s Brook, As yet, no fixed line can be drawn dividing these sub- 
divisions. These millstone grit measures, it may be remarked, are distinguished from the 
corresponding horizon in other parts of the province by their highly caleaceous nature— 
there being numerous beds of limestone, not usually equal in purity to those already noted, 
and the cementing material of the sandstones being often calcareous. 
The Marine Limestones and their associated strata, become obscured as they approach 
the south side of the Coal Field on the east side of the East River, probably by east and 
west faults of great magnitude, similar to those which have on all sides limited the pro- 
ductive Coal Measures by an unconformable frame of Millstone Grit. Approaching 
Sutherland’s River, they reappear and are noted for holding important deposits of 
spathic ore. 
In this district I am not aware of any exposures of the peculiar ‘shell ” limestone of 
Windsor, Shubenacadie, and Brookfield, referred by Sir J. W. Dawson, to Subdivision E 
of the Marine Limestone series, and parallelled by him with limestones belonging to the 
upper part of this section. This limestone is a mass of shells, principally casts, the delicate 
spirals of Spirifer and Athyris being frequently preserved intact. This characteristic lime- 
stone is largely quarried at Brookfield, as a flux for the Londonderry furnaces, and I am 
indebted to Mr. J. Sutcliffe, of the Londonderry mines, for the analysis of it, given further 
on, placed for comparison with one of the same rock from Windsor. 
The analyses which I submit of East River limestones, were made by me sometime 
ago, when engaged in an enquiry into the question of fluxes for the extensive iron ore 
deposits of the district, some of which have been incidentally alluded to in my remarks. 
