EXCURSION IV. 99 



on the north-west coast, and in Cantire, is met with, as are 

 the common slates enveloping the granite nucleus, among 

 which the most remarkable is a very hard and tough banded 

 slate, such as occurs in situ near the line of junction of the 

 granite and slate, in the various localities already indicated. 

 The bands are no doubt due to a re-arrangement of the 

 colouring matter, probably iron, caused by the action of heat 

 along the line of contact. Both parts are equally flinty. 

 Masses of slate also occur composed of quartz and chlorite, 

 chlorite taking the place of mica. This chlorite is of a green 

 colour, and occurs in small very thin flakes, never reaching 

 the size of plates, as mica does. There is very little of this 

 chlorite slate in Arran ; but it occurs in North Bute, alter- 

 nating with mica slate, chiefly towards its outer border. A 

 few masses of another slate occur, which consists chiefly of 

 hornblende, but dashed occasionally with spots of felspar; it 

 is almost like a basalt in structure, but coarser in grain, 

 and more crystalline. It is extremely hard and tough. 

 This is hornblende rock ; it probably forms beds in the slate 

 on the north-west coast, but we know no locality for it. 

 Masses of red sandstone and conglomerate are frequent. 

 The chocolate-coloured Old Red, such as is well seen towards 

 the entrance of Glen Sannox, occurs, and another singular 

 variety, a passage apparently from slate to sandstone. A 

 mass of this will resemble porphyry in one part, in another 

 it will contain pieces of imbedded slate, and again there will 

 be finer layers, like a fine semi-crystalline conglomerate or 

 coarse slate. Large lumps of quartz are abundant, of several 

 varieties; some derived from the conglomerates, and others 

 directly from the northern mountains. 



51. By far the most interesting series of specimens to be 

 found here is that embracing the igneous rocks of later origin. 

 These rocks affect a stair -like arrangement of beds when 

 forming the surface of a district, sheet succeeding sheet 

 upon the ascent of a hill, in a terraced form, like the steps of 

 a stair. They are hence called Trap rocks, from the Swedish 



