125 



not properly a porphyry, for the base ought to be simple. One is 

 speckled with felspar dots, the other more homogeneous and dark- 

 based. Concentric spheroids of greenstone occur, and several varieties 

 of black basalt. A few blocks are met with of a very interesting 

 rock, which we have never found in situ in Arran ; it is intermediate 

 between basalt and pitchstone, and links them together as products 

 of fire generated at different stages of the cooling process. It is hard 

 and tough, and almost homogeneous, like basalt, while it has the 

 semivitreous aspect and colour of a dark blue pitchstone. 



86. We may visit two other objects of interest by the same way- 

 side. Descending from the top of the ridge into the Birk glen, 

 and crossing the burn, we come upon the great pitchstone vein. It 

 crosses the road nearly at right angles, ranging 35 N. of E., and 

 is thirty feet broad. This is not, however, the true breadth of the 

 vein at right angles to its upper and lower surfaces, but merely the 

 extent of the upper surface here stript of its sandstone covering. 

 It is green, of lamellar structure, and contains in parts felspar 

 crystals, giving a porphyritic aspect. Thin white films form on 

 the exposed surfaces, by disintegration; and when in contact with 

 running water it decomposes into a tenacious clay, which forms 

 a thin coating on the surface. The vein is seen in the bed of the 

 burn, of about the same width. The contact with the sandstone is 

 not very well seen ; but there does not appear to be any remarkable 

 alteration upon it. A dike or bed of trap is connected with the 

 vein on the lower side ; but the relation of the two rocks cannot be 

 made out clearly. The other object of interest to which we referred, 

 is a vein or bed of claystone on the east side of the new Lamlash 

 road, almost in the line of bearing of the pitchstone. A quarry of 

 road metal has here been opened upon it; eastwards it can only be 

 traced a very little way, but westwards it is seen below the road in 

 the direction of the pitchstone vein. Its range thus seems to be 

 60 N. of E., or slightly transverse to the other vein, which probably 

 cuts it off, as the claystone is not seen in the river bed on either side 

 of the pitchstone vein. In the quarry the claystone splits below 

 into rhombic blocks ; in the upper part it is slaty. We have referred 

 to this quarry already (Art. 57, p. 83); and also alluded to the 

 many claystone beds on the north front of the ridge of Dun-fion. 



We may return by the new Lamlash road; but there is more 

 variety to be met with in descending the stream, and passing through 

 the ornamental grounds connected with the hotel. The sandstone 

 strata are well exposed, and there are some curious dikes. 



