Ordnance survey of the county of Londonderry / Colonel Colby ... superintendent. Volume the first.

  • Ordnance Survey of Ireland.
Date:
1837
Catalogue details

Licence: Public Domain Mark

Credit: Ordnance survey of the county of Londonderry / Colonel Colby ... superintendent. Volume the first. Source: Wellcome Collection.

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    (but not always) co-incident with the principal cleavage, or plane of stratification, and they occasionally project in sharp edges beyond the face of the secondary cleavages, which, were it not for these inter- ruptions of their continuity, might from their general smoothness be considered planes of stratification. It is difficult to state any exact proportion in quantity as existing between these rocks, hut an approxi- mate estimate may be given as follows:— 1. Mica Slate, passing into Gneiss.—This is of small extent, protruding amidst the mere schistose strata, and exhibiting a rugged aspect, and almost massive structure. In texture also this slate is remarkable—small isolated lumps of bluish quartz, of the size of, or less than a pea, being arranged in layers parallel to the laminae of the schist, and giving to the cross fracture a granular, or even a conglomerate appearance. The paste in which they are set is common quartz, mixed occasion- ally with felspar, the crystals of which become frequently prominent. In this, as in all the other rocks, the mica can only be discerned on the face of the laminae, and is almost evanescent. The hill of Ballougry may be cited as a good example of this rock. It there alternates with the common variety, in beds 30 feet thick, and terminates abruptly on the summit of the hill. 2. Mica Slate, passing into Clay Slate.—The distinction of the mineral substances is here almost, or entirely lost. This principally occurs towards the S. of the parish. Judging by the position of the localities, where this variety is visible on the surface, it occupies a strip a mile wide, extending across the parish. It is succeeded by 3. Mica Slate, passing into Quartz Slate—The quartz, in thin layers, is the most conspicu- ous material, and the mica a mere evanescent parting, which from its frequent recurrence renders the rock highly lamellar. This is the prevailing rock of the parish, occurring on the S. as well as N., and occupying at least two-thirds of its surface. In structure it is variable, and also in composition, the mica being replaced by talc, and probably by chlorite, though, from the extreme thinness of the partings, it is difficult to distinguish between them. The undulations of surface, so frequent in mica slate, may be noticed in the two last varieties, the laminae being twisted in a very curious manner. Sometimes, indeed, a break occurs in the middle of one of these minute bends, the general direction of the laminae being preserved above and below it. And it is remarkable that such contortions, accompanied by a tendency to break into small angular fragments, are more frequent at the surface than below it—the continuity of the laminae extending, and the quality of the stone for building improving, at the depth of about 10 feet. Quartz veins are common in the first, or gneiss, variety, rare in the second, and not very common in the third. A uniform character prevails to a considerable extent in all, namely—that small specks of oxide of iron are visible on the cross fracture. None of these rocks attain any great elevation—the highest point in the parish, Holywell Hill, which is composed of the second variety, being only 860 feet above the sea. Subordinate to the preceding formation are occasional beds of limestone and greenstone. The limestone appears only on the E. of Mullcnnan, in a small excavation at the base of the little lime-kiln on the S. of the old mill, and close to the river. It is dark in eolour, finely granular, and analogous