6 S. Allport — On the Pitchstones of Arran. 



with belonites. Tliis stream-like arrangement of the smaller par- 

 ticles clearly indicates that the mass of the rock remained in a viscid 

 state after the formation of the crystals; in all probahility it is 

 mainly due to the flowing movements of the lava previous to its 

 consolidation. A similar structure is common in trachytic, and other 

 volcanic rocks, and also occurs to a slight extent in some of the older 

 melaphyres. 



The varieties of texture in the striped pitchstones are extremely 

 interesting, and deserve careful study, as they j)resent peculiarities 

 not to be found in other kinds of igneous rocks. In a specimen 

 taken from a dyke about a mile south of Tormore, the striped 

 appearance is due to the occurrence of numerous parallel bands and 

 lines of a lighter colour than the mass of the rock, and an examina- 

 tion of thin transparent sections shows not only the nature of the 

 bands, but that they also exist in microscopic proportions ; for in one 

 section, three quarters of an inch in diameter, there are no less than 

 twenty-two light and dark coloured bands alternating with each 

 other. The dark bands consist of a pale brown glass, containing a 

 few tuft-like and stellate groups of small green crystals, while the 

 pale stripes are of clear colourless glass, in which are to be seen 

 numerous belonites and small pellucid granules ; these are, however, 

 much larger and fewer in number than the mierolites forming the 

 colouring matter of the brown bands. There is no tendency to 

 cleave in the direction of the bands, and it is evident that the struc- 

 ture is quite different from the foliation of the crystalline schists ; 

 the rock is simply a continuous homogeneous glass, in which the 

 devitrified or crystalline particles have' arranged themselves in 

 alternate layers characterized by differences in texture only; minera- 

 logically, there is no difference whatever. Fig. 2 is a very success- 

 ful attempt to represent the structure here described. A banded 

 pitchstone from a dyke in the granite south of Cior Mor has a very 

 similar structure. 



A short distance from the dyke just described there is another 

 of great interest ; it is sixteen yards wide and strikes in a N.N. W. 

 direction from the sea across the shore, and then rises straight up 

 the face of the cliff through the sandstones, which form a smooth 

 wall on each side. The west side of the dyke contains a band 

 of liornstone, one foot thick, in contact with the sandstone, then 

 several feet of pitchstone, the remainder being basalt ; in one place 

 a strip of sandstone has been caught up between the basalt and 

 pitchstone and baked to a hard quaitzite. This pitchstone exhibits 

 a very different texture from the one just described ; it consists 

 of a yellowish-brown base, in which a few acicular pyroxenic crystals 

 are arranged in stellate groups at some distance apart. Like others 

 previously described, these groups are bordered by a clear space 

 surrounded by the coloured glass, which forms a re-entering angle 

 between the rays, the appearance being that of a bright star of five 

 or six pointed rays. See Fig. 3. 



The liornstone so frequently mentioned by Bryce in his " Geology 

 «f Arran " is simply altered pitchstone ; a specimen taken from the 



