in Sutherland, Ross and the Hebrides. 359 



phyritic greenstone, bifurcates upon reaching the summit of the cHff*. Pro- 

 fessor Sedgwick and myself have since observed, that the extremities of 

 the horizontal prisms of which the larger branch of this dyke is composed, 

 consist of a dark green pitchstone, forming a band of about four inches in 

 thickness, which, where not disintegrated, is seen in contact with the fossil 

 limestone ; the latter being apparently unaltered in its character. I am 

 not aware that the existence of pitchstone in association with so recent a 

 formation in Scotland has been hitherto noticed, and the fact is of importance 

 in assisting us to limit the antiquity of those trap rocks which contain 

 that mineral. In the Isle of Arranf it is extensively developed in the new 

 red sandstone, from which formation it appears to have extended upwards 

 through all the beds of the oolitic series in the Hebrides ; for we again met 

 with it at Carsaig on the south coast of Mull, forming the principal part of 

 one of the numerous trap dykes, which there traverse the lias and inferior 

 oolite. This latter dyke is about four feet in width, and is formed of hori- 

 zontal prisms, the extremities of which consist of thin coats (two inches thick 

 each) of blueish compact felspar rock, followed on either side by broader bands 

 of greenstone, the centre being- composed of about two feet of dark green por- 

 phyritic pitchstone ; some veins of which penetrate through the exterior 

 zones. Another trap dyke on this shore cuts through the lias, indurating 

 the beds in contact, and sending out numerous veins, one of the largest of 

 which envelops fragments of the lias shale including gryphites and belem- 

 nites. In this and many other situations, particularly in the cliffs east of Loch 

 Buy J, the presence of the trap seems to have converted the lias into Lydian 

 stone, and the sand of the inferior oolite into a compact siliceous rock ; 

 whilst, in an equal number of cases throughout the Hebrides, the intrusive 

 rock traverses the beds of these same deposits without producing the slightest 

 alteration in them. The altered masses, however, chiefly predominate in 

 those places where the masses of basaltic trap are of the greatest magnitude ; 

 as on the south coast of Mull, at the Inimore of Carsaig, or at the mouth of 

 Loch Buy, and on the north-eastern coasts of Skye. 



The organic remains collected on this last occasion in various parts of the 

 Western Islands §, comprise many characteristic fossils tending to confirm 

 the comparison already instituted, and to identify these deposits with the 

 oolitic series and lias of English geology. If assiduous fossilists could devote 



* Ante, page 310 ; and see PI. XXXI. fig. 4. 



t A memoir has recently been read before the Geological Society on the Secondary Strata of 

 the Isle of Arran, in which Professor Sedgwick and myself have assigned the sandstone of the 

 south-eastern and southern coasts of that island to the new red sandstone. 



X See PI. XXXV. § See the Supplemental Table, page 365. 



