Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/189

 shale and sandstone, and, as we may conclude from the general bearings of the strata already described, occupy the upper beds of this formation. It is impossible to say that they do not exist at a greater depth, since the inferior strata, as I have already shown, can scarcely be considered sufficiently accessible to enable us to determine on the absence of a substance of which the thickness does not exceed a very few inches. Nevertheless, no indications of coal can be perceived along the eastern line of cliffs, where the deeper strata are exposed, and we may therefore for the present conclude that they lie above the lias and its associated sandstone, or at least among its uppermost beds. It can serve no purpose to enumerate the places where these indications of coal have been observed, since they cannot be verified on the map, and are indeed generally nameless. As the strata rarely exceed an inch in thickness, it is equally evident that those which are visible are worthless in an economical view, while the certainty of a speedy interruption from the intrusion of trap removes all temptation to penetrate to greater depths, or to expend capital in a more effectual research.

While on the subject of coal, I may add to the former account, that I have observed portions of wood coal in more places than those originally enumerated, but they are no where of sufficient importance, or marked by any such peculiarities as to require further description.

In describing the several trap rocks of Sky, I am sensible of having often spoken generally, when the circumstances might perhaps have admitted of more accurate details. The cause of this however is principally to be sought in the imperfect acquaintance which geologists still possess with this infinitely varied and obscure class of rocks, an obscurity which increased experience is daily