994 THE CARBONIFEROUS Of CAPE BRETON— GILPIN. 
As compared with the productive measures, these strata show 
a much larger percentage of sandstones, frequently coarse and 
sometimes conglomeritic. There are fewer argillaceous layers 
and much false bedding. Near the old syenitie and felsitic rocks 
the prevailing color is red ; further away, where the material has 
been derived from the preceding Carboniferous horizons, gray 
shades are met. The formation is also distinguished from the 
Productive and the Marine Limestone series by the absence of 
calcareous matter. Numerous coal seams are met, some of which 
are persistent over long distances and of workable dimensions. 
Others are not at present considered of value in the presence of 
the large seams now worked, but will prove in the future an 
important source of coal. This series stretches from the Mira 
River to the Eastern shore of Sydney harbour, and then widens 
until it oceupies nearly all Boularderie Island. The maximum 
thickness in this district is 5,700 feet, but it rapidly diminishes 
to the Northward, until at Cape Dauphin only 500 feet is 
exposed. 
A long arm of millstone grit extends up the Salmon and Mira 
Rivers and overlaps unconformably the marine limestone and 
basal conglomerates, and rests against the Mira and East Bay 
felsites. The underlying divisions of the carboniferous erop at 
various points throughout the district, and extend irregularly 
through Loch Lomond and Grand River to St. Peter's. 
This outlier presents the outerops of several small seams of coal 
apparently underlying a large extent of ground. There has not 
been any attempt made to find other seams, or even to test the 
value and extent of these outcrops. The measures including the 
coal seams possibly represent the upper part of the millstone grit 
as exposed to the eastward of the Productive Measures of the 
Sydney Coal Field, and are on a horizon corresponding to that 
of part of the millstone grit lying south of Sydney town, where 
similar outcrops of coal are found. 
The Marine Limestone formation occupies a triangular tract 
of ground between the arms of Sydney harbor, and attains a 
thickness of about 2,000 feet. It is composed principally of red 
and gray shales, sometimes approaching marls in aggregation, 
