22 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
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. 1 
ft is stated that the toadstones, though subject to great variations in thick- 
ness, are never seen to cut across the limestones. 2 But I suspect that proofs 
ol 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 
loadstone of that valley, which is undoubtedly a true lava-how, 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. 3 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 
ot leak Forest, where the limestone above a coarsely crystalline dolerite has 
been converted into a white saceharoid marble for about two yards from the 
junction. 
3. THE ISLE OE MAH 
Bising from the middle of the Irish Sea, within sight of each of the 
three kingdoms, with a history and associations so distinct, yet so intimately 
inked with those of the rest of Britain, this interesting island presents in 
its geological structure features that connect it alike with England, Scot- 
land and Ireland, while at the same time it retains a marked individuality 
m regard to some of the rocks that form its framework. Its great central 
ridge of grits and slates, which still rises 2000 feet above the sea in the 
summit of Snaefell, must have formed a tract of dry land in Carboniferous 
time, until it sank under sea -level, and was buried beneath the Carbon- 
iferous and later formations. Along the southern margin of this ancient 
land, a relic of the floor of the Carboniferous sea has been preserved in a 
small basin of Carboniferous Limestone which covers about seven or eight 
square miles. This remnant has a special interest in geological history, 
for it has preserved the records of a series of volcanic eruptions which took 
place contemporaneously with the deposition of the Carboniferous Lime- 
stone. 
The geology of the Isle of Man was sketched in outline by J. F. 
Berger, J. Macculloch, 5 and J. S. Henslow, 0 and was afterwards more fully 
illustrated by J. Q. Cumining. v To the last-named observer we owe the 
1 North Derbyshire Memoir, p. 23. 2 123 
J. M. Mello, Quart. Joum. Geol. Soc. vol. xxvi. (1871), p. 701. 
Trans. Geol. Soc. 1st ser. vol. ii. (1814), p. 29. 
5 Western Islands of Scotland (1819), vol.’ii. p. 571 . 
6 Trans ■ Ge ° l Soc ■ lst ser - V- (1821), p. 482. ' 7 The Isle of Man (1848), chap. x. 
