3oo DEVONIAN DOLE RITES. 



coloured felstones, and various other felstones follow, interstratified 

 with ash and agglomerate. 1 



The prevailing porphyrite extends into Peeblesshire, Lanark, and 

 Ayr. It forms the greater part of the hills between the Lyne Water 

 and the Clyde, but the true bedded character is less marked there than 

 in the Pentland Hills. The porphyrites thicken to the S. W. towards 

 Symington, where several necks of felstone probably mark volcanic 

 vents. 2 



Devonian Volcanoes of Cornwall. Prior to the upheaval of 

 the great masses of granite of Devon and Cornwall, and during the 

 deposition of the Devonian rocks which those upheavals disturbed, 

 enormous quantities of doleritic lavas were poured out from vol- 

 canoes in the Cornish area. The recognition of the igneous rocks is 

 not always easy, because the old slates are frequently so metamor- 

 phosed as to put on the characters of doleritic rocks ; and trap rocks 

 and ash beds so graduate into slates that the change is almost im- 

 perceptible Their history has been unravelled almost entirely by Mr. 

 John Arthur Phillips, F.R.S. The largest group of these rocks is 

 situate in the neighbourhood of Penzance, where they consist of a 

 series of fissile greenish slates, containing compact crystalline beds 

 without trace of lamination. Several beds are well seen around the 

 shores of Mount's Bay. 



Those slates which have undergone the least metamorphism 

 consist of crystalline felspar or diallage, magnetite, titanic iron, and 

 occasional specks of pyrites, prisms of apatite, and flakes of brown 

 mica. The more altered rocks consist of a colourless transparent 

 base, through which hornblende and viridite are diffused, with 

 pseudomorphs of augite and decomposed felspar. On the eastern 

 side of the Valley of Tollarn the large crystals of felspar are well 

 preserved. The rock at Battery Point, and the Chapel Rock, 

 are hornblendic lavas ; but Mr. Phillips regards the hornblende 

 as being, sometimes at least, a product of metamorphism. These 

 rocks contain from 43 to 47 per cent, of silica, 18 to 21 per 

 cent, of alumina, 9 to n per cent, of ferrous oxide, 6 to 12 

 per cent, of lime, 4 to 7 per cent, of magnesia, and i to 3 or 

 4 per cent, of potash and soda. The composition of the killas, or 

 Devonian clay-slate, is often almost identical, and though the 

 percentage of silica is sometimes higher, it may also be lower. 

 Other altered dolerites occur in the Gurnard's Head; and the 

 headlands at the extreme limit of Porthglaze Cove are so changed 

 that apatite is the only unaltered mineral remaining. In the St. Ives 

 Bay district the rocks are very similar to those in Mount's Bay, and 

 it is just as difficult to distinguish whether the dark mineral in the 

 rock was augite or diallage. There is usually present some granular 

 quartz and a little viridite. Near Camborne an igneous band occurs, 

 stretching further east to South Koskear, which is known as blue 

 elvan, and consists of garnets and axinite. Two miles west of 



1 Geikie : Mem. Gcol. Surv. Scot. Edin., 1861. 



2 Mem. Geol. Surv. Scot. ; Explanation of Sheet 24. Arch. Geikie. 



