Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/137

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this chain sends forth; the two latter are feeders of the Baun, the first empties itself into Lough Foyle.

The geological constitution of this third system is highly important and interesting, it is as has been already observed, wholly secondary and uniformly covered by enormous stratified masses of basalt; this covering appears to acquire its greatest thickness on the north. The basaltic cap of Benyavenagh, the most northern summit of the western chain measuring more than 900 feet; and that of Knock-lead, similarly situated in the eastern chain 980 feet: the average depth of this superstratum may therefore be safely estimated at 545 feet, and its superficial extent at 800 square miles, a solid mass of extraordinary and imposing dimensions.

In the strata underlying the basalt. the English geologist is agreeably surprised to recognise many of the most important of those formations, which reposing upon the coal measures, occupy such an extensive tract in the south and eastern counties of his own island, but which in Ireland are entirely confined to the comparatively small district now under consideration, newer extending far beyond the circumference of the great basaltic area; a circumstance which almost naturally leds to the conjecture that they may have been originally much more extensive, but have been elsewhere removed by the agency of some destroying and denuding force, to which in this quarter alone an effectual resistance was opposed by the firm and massive suprestratun of basalt which covered and protected them.

The beds alluded to occur in the following order, proceeding from beneath the basalt downwards.

1. Chalk.─This formation, which in England cannot be estimated at less than 800 feet in thickness, does not in Ireland average more than 200. It agrees exactly with the lower lads of the English