324? PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Ground plan of a Dike on the N.W. shore of Langness. 

 S cale 30 yards to 1 inch. 



E 



Note. — The shaded parts are Old red sandstone. 



they vary also themselves in general character according to the na- 

 ture of the strata with which they are in contact. We have also 

 extensive traces of igneous action where the trap itself is not imme- 

 diately developed, and this is particularly the case along axes of 

 elevation and lines of fault. All these appearances may perhaps 

 suggest the idea of heated gases charged with acids escaping 

 through the cracks formed during some of the periods of convul- 

 sion. But I would observe, that in several instances the crystalline 

 character of the limestone, and its tendency to break up into regu- 

 lar geometrical figures, are distinctly attributable to the continued 

 action of the cooling of a fluid mass of trap, forced in between the 

 Old red conglomerate and the limestone, or rather, perhaps, forced 

 in under great pressure amongst the looser and separable beds of the 

 conglomerate. We may almost suppose that the entire district must 

 have been floating upon a molten sea of igneous rocks, subject to 

 many convulsions, frequently exerting an upward pressure at many 

 points, but at the same time held down by a tough mass of lime- 

 stone resting upon moveable and permeable beds of conglomerate 

 of very moderate thickness. A reference to the Map denuded of the 

 tertiary deposits will show that it is crossed by a series of undula- 

 tions and studded with bosses (see Map, PI. XV.). 



As the general strike of the more important of these undulations 

 is nearly coincident with that of the great mountain-chain of the 

 island, which is about 80° west of magnetic south, it appeared to me 

 at one time that they must have been produced by lateral pressure 

 at the time of the elevation of the central axis of the island, and that 

 the bosses were caused by the intersection of the more prominent 

 cross-undulations created at the same time as the axis of disturb- 

 ance which runs from Ballasalla about 10° west of magnetic north. 

 And this solution may perhaps hold good in whole or in part, so far 

 as the main features of the district are concerned ; but my attention 

 having been especially directed to the examination of the very sin- 

 gular bosses at Skillicore, between Ronaldsway and Coshnahawin 

 Head (Plate XVI.), and another series at Poolvash, I am rather in- 



