Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/456



PLATE 10, 10*. Sectional views along the N. E. coast of Ireland.

PLATE 11.

Sectional views of Kenbaan head and bay, on a larger scale than in the former plate.

PLATE 12.

Ground plan and section of Birch-hill colliery, near Walsall, Staffordshire.

PLATE 13. Map of Glen Tilt.

The Map exhibits the general appearance of the rocks which are visible at the surface. These are distinguished by colours, of which an explanation is given in the margin. I may remark that the same colours are used to represent the same substances throughout the whole of the plates belonging to this paper.

The Map does not pretend to give every rock which comes to the surface, since the spaces which many of them occupy are so small as to have rendered such a detail impracticable. I have omitted particularly many of the small masses of quartz rock which are visible on the granite, as they would, instead of elucidating, have obscured the explanation which this map is designed to give. The southern side being of a more simple construction admitted of a more real detail, yet in this also I have not pretended to lay down the perpetual and often minute alterations of the schistose rocks with the limestone, since there would not have been room for this purpose: I have contented myself with indicating them in a general way.

In the method used in colouring, I have defined each colour in those places where the rocks themselves are visibly defined. Where these boundaries are uncertain from the covering of soil or other causes, the colours are undefined. The uncoloured parts which lie near the river are intended to represent the alluvial matter, although there is little doubt that the junction of the limestone and the granite exists below it. I have detailed as well as I could the several points where that junction is actually visible: greater accuracy would have been impracticable on the map which I was obliged to make use of, with which my own measurements were often at variance. But it is a matter of no moment for the purposes of this paper, since its object will be equally accomplished whether there are twelve or thirteen junctions visible, or whether Forest lodge is three or four miles from Gow's bridge. I have only marked one or two masses of porphyry, as a knowledge of their places was of no moment.