GUMMING ON THE ISLE OF MAN. 333 



raneously with the deposition of volcanic ashes ; and that subse- 

 quently the whole series so formed has been subjected, under pres- 

 sure, to the action of considerable heat. 



The sections which I have made are based on this supposition, 

 and offer the nearest and simplest approximation to order which I 

 could discover, amidst so much contortion and breaking up of the 

 strata and intrusion of volcanic products. The older dark lime- 

 stones of the basin (which we are here enabled to measure to the 

 thickness of 130 feet) are suddenly thrown downwards towards the 

 line of disturbance running north-westward from the Stack, and are 

 intersected by several trap-dikes also running north-west (see PI. XV. 

 and section, PI. XVI.). Above them we have the Poolvash limestones 

 nearer the Stack, greatly altered and crystalline ; then a series of 

 trap-tufF, trappaceous limestone, ^^c; and overlying and unconform- 

 able to all these are other broken and twisted beds, and an out- 

 pouring of trap-tufF with mingled fragments of the broken strata. 

 The breaking up of the beds appears to have been occasioned by 

 the eruption of the trap, which forms the basaltic pile of the Stack, 

 and which may perhaps be only the more prominent portion of the 

 largest trap-dike, assuming that columnar structure in consequence 

 of some peculiar condition of cooling. 



The whole distance hence to Poolvash Bay, wherever the beds 

 are uncovered from the drift, exhibits this remarkable trappacieous 

 formation ; and as we proceed onward in that direction, the order of 

 the formations appears to come out with greater regularity, and the 

 different beds attain a greater thickness. In some of the deeper 

 chasms which the action of the sea has worn in the tuff, the Posi- 

 donia schist is seen ; and where we come upon the bay itself, the 

 shore turning suddenly to the northward and north-eastward, we 

 find that large development of it which is worked as a marble 

 quarry. In many places the schist rests directly on the limestone, 

 and thins out around masses of the limestone in such a manner as 

 to indicate that it was deposited after some disturbance of the lime- 

 stone had taken place, and that the sea-coast was in this immediate 

 neighbourhood. We may indeed almost fancy we can trace the 

 ancient outline of the land ; and the presence of the fossil ferns 

 found in one of these recesses appears to indicate the same fact. 



Westward of Poolvash the trappaceous deposits do not appear, and 

 the shore is so much intersected with trap-dikes and the rocks are 

 so much altered, as to prevent all description of that neighbourhood. 

 The hill above Balladoole appears to have been raised in a dome, 

 and it is not unlikely that there may be caverns formed in that 

 neighbourhood, as there is close alongside the great fault (which 

 has before been mentioned as bringing up the dark beds of the 

 lower series of limestone), a strong spring of salt water wliich con- 

 tinues to run several hours after ebb-tide, apparently from an un- 

 derground pool which is filled at high-water. At the mouth of the 

 stream beyond the farm called Copenhagen, where the high road 

 comes upon the shore, we find ourselves upon the lower scries of 

 limestone, the newer or Poolvash scries having suffered denudation 

 to the north-west of the stream. 



VOL. 11. — PART I. z 



