S. AUport — On the Pifchsfones of Arran. 7 



side of this dyke exhibits tinder the microscope the characteristic 

 structure of the unaltered rock ; the quartz crystals are the same 

 in every respect, but the felspar is no longer transparent, and the 

 clear green belonites have lost their colour, and appear as yellowish- 

 brown specks. 



On the south-east slope of Goatfell, above the woods surrounding 

 Brodick Castle, there are some blocks of a magnificent porphyritic 

 pitchstone, of a very different character from any of the preceding. 

 They lie in the bed of the carrier, which conveys water from the 

 mill dam. The rock was not found in situ, as the ground is here 

 very unfavourable for examination, the mountain side being covered 

 with debris from the higher granitic masses and thickly coated with 

 turf. 



The matrix is a greenish-black glass of dull lustre, crowded in 

 every part with large crystals of glassy felspar, some of which are 

 half an inch in length. Quartz crystals are also abundant. As the 

 base is very finely granular, it requires a power of 500 for its ex- 

 amination, and is then seen to consist of a colourless transparent 

 glass crowded with belonites of nearly uniform size ; they are straight 

 prisms T-gL- of an inch long by -j-o-^o-o broad, and intermingled 

 with them are numerous minute granules. 



These microlites are very regularly distributed throughout the 

 glass, and lie at all angles, except when near one of the larger 

 crystals, when their long axes at once assume a uniform direction, 

 and they aj)pear to bend round it. There are in addition many 

 small crystals of augite about the ^^-^ of an inch long, and a few 

 grains of magnetite. Lastly, there are the large felspar and quartz 

 crystals, and a few of augite porphyritically imbedded in the matrix. 



The felspar is chiefly triclinic, but orthoclase is also present. The 

 crystals are, as usual, penetrated by the augite and magnetite, but in 

 this rock they are especially remarkable for the large amount of the 

 matrix inclosed in their cavities. Some are little more than skeletons 

 of felspar, the whole of the interior being completely honeycombed 

 with cells filled with the matrix just described, or with a brown 

 glass, the latter sometimes predominates. Fig. 6 is a crystal of tri- 

 cKnic felspar, as seen in a thin section. During the process of 

 grinding, the mass of the crystal was found to be similarly filled 

 with brown glass. 



In other crystals the cavities are arranged parallel with the sides, 

 forming a broad band near the surface, the central part being clear 

 felspar with only a few detached cavities. 



It is evident that the chemist would be completely misled in an 

 examination of these crystals, and it may be well to point out that, 

 although this is certainly an exceptional case as regards the quantity 

 of foreign matter, still the occurrence of various minerals in the 

 constituents of igneous rocks is so common, that it is quite sufficient 

 to account for many of the discrepancies to be found in the lists 

 of analyses. 



Having examined the most marked varieties met with in Arran, 

 it may be well to add a description of a porphyritic pitchstone from 



