Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/33

Rh the Shropshire side of the river Dee, and three miles S.E. of Pentre Isaf, where they were first seen. Their position is shown on both the horizontal and vertical sections.

Starting with the Spirorbis-limeatone as our base-line, we have, in ascending order, first a series of red and blue rocks, shales, green and brown sandstone, and grit rocks, with ten thin coal-seams and their underlying clays. The total thickness of this series, from the Spirorbis-limestone to the topmost coal-seam, is 220 feet. I may here remark that, with some variations, the same strata have been proved in various sinkings and borings in the neighbourhood.

Above this group of strata we have next about 300 feet of green and greenish-grey rocks with brown and grey sand and gritstones, with pebbly conglomerated and brecciated beds, usually of a greenish colour. These are interstratified with red marls and clays. The green rocks present a great variety of texture; some are fine-grained sandstones, others rougher grits passing through fine and course breccias into conglomerates. In the Ifton section the fragments and pebbles do not exceed three inches in length by one inch and a half in breadth. It is, however, probable that if we could follow these deposits westward to what would be their shore-line, we should find the pebbles and angular fragments increased to boulders and blocks of a larger size. These beds are also exposed along the banks of the rivers Ceiriog and Dee; and there we find that the breccias and conglomerates are not continuous over a large area, but form oval, wedge-shaped, and irregular masses in the midst of the finer sandstones and marls.

They thus present the appearance of deposits made in a shallow sea subject to currents, eddies, and storms. The pebbles and fragments consist chiefly of Lower or Cambro-Silurian rocks with their imbedded quartz, felspar, greenstone, and porphyry, together with fragments of Wenlock Shale and Carboniferous Limestone, The whole series indicates a source in the hilly region about Glynceiriog and Llanarmon, which lies from ten to twenty miles west of their present position. Associated with these breccias and conglomerates are drifted plant-remains.

It will be seen by a reference to the horizontal section that this series of beds thins out before it reaches the shafts of the Brynkinallt Colliery on the west; nor is it present, except in an attenuated form, in the Hafod-y-bwch shafts of the Ruabon Colliery, five miles to the north.

This series of strata is overlain by another of nearly equal thickness consisting of red, white, purple and variegated marls. These marls are interstratined in the lower part of the series with red and brown sandstones. In the middle portion there are numerous thin bands of grey calcareous rock. In the upper marls are numerous calcareous clayey concretions charged with crystals of sulphide of iron.

There are two thin layers of carbonaceous matter in this series—one near the base, in which I have detected traces of Calamites, and one near the middle of the series, which forms a true coal-seam