S. AlJport — On the Igneous Rocks of Arran. 537 



Specimens of pitchstone collected in Monamore Glen and Birk 

 Glen, near tlie old Lamlash road, afford beautiful varieties of the 

 rock represented in Plate I., Fig 1, combined with the structure of 

 Fig. 2. A dark green pitchstone from Invercloy contains numerous 

 crystals of quartz and felspar, which give it a porphyritic character ; 

 the base is a clear colourless glass, thickly crowded with very short, 

 straight belonites, which wind in streams round the larger crystals, 

 but there is no arrangement in groups. Some of the quartz crystals 

 contain remarkable cavities inclosing small fern-like groups of 

 belonites, although there are none whatever in the surrounding 

 matrix. 



A specimen of black pitchstone deserves special mention. It was 

 taken from a boulder on the high ground near West Benan ; two 

 other boulders of the magnificent porphyritic pitchstone previously 

 described, together with many others of the coarse-grained granite, 

 were lying near it ; the whole assemblage being evidently derived 

 from the Goatfell granitic range. 



It may be well to observe here, that of the hundreds of boulders 

 scattered over this part of the country, I did not see one of any rock 

 foreign to the island. The specimen in question is quite black, 

 of irregular brittle fracture, and contains a few crystals of felspar 

 and brown augite. The microscopic structure is highly remarkable, 

 and differs considerably from any previously described. With a low 

 power, the colourless glassy base appears to be crowded with 

 numerous groups of slender black prisms, exhibiting the follow- 

 ing singular arrangement. A long acicular prism forms an axis, 

 to which are attached, on opposite sides, two rows of similar 

 shorter prisms, projecting from it at right angles, like the teeth of a 

 double comb. With a higher power, the prisms are easily recog- 

 nized as the usual pyroxenic belonites thickly studded with ex- 

 tremely minute grains of magnetite; the rock itself is rather strongly 

 magnetic. Well formed crystals of orthoclase and plagioclase are 

 imbedded in the base, together with many distinct characteristic 

 crystals of augite. Quartz appears to be absent from this rock. 



One of the porphyritic pitchstone boulders just referred to is also 

 of great interest, as it contains a considerable proportion of fine- 

 grained basalt included in it. Under the microscope, the pitchstone 

 itself is seen to be quite the same as that previously described (p. 7), 

 but it contains, in addition, small isolated fragments of basalt, while 

 the basaltic part of the rock incloses quartz crystals, which have 

 included in their cavities portions of the basaltic matrix in which 

 they are imbedded. The basalt is very fine-grained, and consists 

 almost entirely of small, clear, felspar crystals, and very minute 

 grains of magnetite. A few small grains of augite may also be 

 detected. 



It has already been explained that the characteristic base of pitch- 

 stone is a homogeneous glass loithout a trace of dotible refraction, and 

 therefore remaining dark between crossed Nicols ; the base of 

 felsites, porphyrites, and other allied rocks, is, on the other hand, 

 characterized by a felsitic structure, and a felsitic base invariably 



