I did not observe thin partings of tuff and disseminated volcanic lapilli among the limestones, such as are so marked in the Lower Carboniferous formations of West Lothian, and in the Limerick basin, to be described in the following chapter. But a diligent search might discover examples of them, and thus prove that, besides the more prolonged and continuous eruptions that produced the thick bands of tuff, there were occasional feeble and intermittent explosions during the accumulation of the thick sheets of limestone. Some of the layers of "red clay" observed in shafts sunk for mining purposes may perhaps represent such spasmodic discharges of fine fragmental material. 5. THE SILLS.--No attempt has yet been made to determine whether and to what extent the toadstone bands include true intrusive sheets. My own brief examination of the ground does not warrant me in making any positive statement on this subject. I can hardly doubt, however, that some, perhaps not a few, of the toadstone bands are really sills. In the accounts of these rocks contained in the mining records a distinction, as already remarked, appears to have been generally drawn between "toadstone" and "blackstone." The latter term is applied to the black, fresh, more coarsely crystalline, and generally non-amygdaloidal rocks, which, so far as I have been able to examine them, have the general external and many of the internal characters of the Carboniferous sills of Central Scotland. At Snitterton near Matlock one of these "blackstones," as already mentioned, is said to have been found to be 240 feet thick.[45] [Footnote 45: _North Derbyshire Memoir_, p. 23.] It is stated that the toadstones, though subject to great variations in thickness, are never seen to cut across the limestones.[46] But I suspect that proofs of intrusion and transgression will be found when diligently sought for. It appeared to me that the dark, compact, crystalline dolerite, which was formerly quarried in the middle of Tideswell Dale, may be separated from the vesicular toadstone of that valley, which is undoubtedly a true lava-flow, and that it does not always occupy the same horizon there, being sometimes below and sometimes above the amygdaloid. Where it rests on a band of red clay the latter rock has been made columnar to a depth of nine feet.[47] Alteration of this kind is very rare among the Carboniferous bedded lavas, but is by no means infrequent in the case of sills. But the most important proof of alteration which I have myself observed occurs at Dale Farm near the village of Peak Forest, where the limestone above a coarsely crystalline dolerite has been converted into a white saccharoid marble for about two yards from the junction. [Footnote 46: _Op. cit._ p. 123.] [Footnote 47: J. M. Mello, _Quart. Journ. Geol. Soc._ vol. xxvi. (1871), p. 701.] 3. THE ISLE OF MAN