81 



After measuring with care the directions and breadths, and noting 

 the characters of forty-four dikes, chiefly of greenstone, between 

 Brodick and Lamlash, and also those at Tormore, it did not appear 

 to us that any other dependence of the direction of these dikes upon 

 the local centre of the granitic eruption could be traced." (Man. 

 ofGeol, 1855, p. 505.) 



Kocks of the felspathic types, which are most closely allied to 

 granite, seem to have no more intimate relation to the granitic centre 

 than have those of the hornblendic. The largest body of porphyry 

 on the island is that of Leac-a-breac, on the south-west; the 

 next in extent is that of Dunfion, over the Corrygills shore. A 

 similar rock occupies a small space on the Windmill hill, over Grlen- 

 cloy, in close connection with the Ploverfield granite. A different 

 variety forms the bold precipices of Drumadoon, on the west, and the 

 principal mass of Bennanhead, on the south. The largest body of 

 claystone forms the middle and upper portions of Holy Isle, and has a 

 thickness of nearly 900 feet ; extensive beds and dikes of the same 

 substance are met with in Lamlash river and at Blackwater-foot ; 

 lesser veins and beds in many other places. All of these cut through 

 or overlie the carboniferous formations of the southern section of the 

 island ; the northern section, the region of granite, slate, and old red 

 sandstone, is almost devoid of these felspathic rocks ; a few dikes only 

 are met with ; almost all those in this tract being of hornblendic rocks. 

 The pitchstones also, exclusive of those in the granite, are almost 

 all met with in the neighbourhood of Brodick, and towards Mauchrie 

 water on the opposite shore. These various felspathic rocks thus 

 seem to correspond pretty nearly on opposite sides of the island, and 

 to have no relation, in their position, to the granitic nucleus. Neither 

 does there seem to us any good foundation for a generalization put 

 forward by some writers on Arran, that rocks of this type are more 

 abundant on the western than on the eastern side of the island. 



Overlying rocks purely hornblendic, as basalt and greenstone, 

 occur chiefly over the central and south-eastern portions of the 

 southern plateau, south of the parallel of Lamlash. North of this 

 line too great an extension has been hitherto given to these rocks ; 

 they merely cap the sandstone in isolated knolls or narrow bands of 

 inconsiderable thickness. The details regarding these, as well as the 

 felspathic rocks, will be seen upon the map, and will be more fully 

 noticed in the several " Excursions" which follow. Under the same 

 head we shall notice the changes made by the dikes on the adjoining 

 rocks. 



In Arran, as elsewhere, almost all the dikes are simple that is, 



M 



