2 >S'. Allport — On the Pitchstones of Arran. 



and porphyritically imbedded. A glassy material of similar character 

 also occurs not unfrequently in basaltic rocks of all ages, but in much 

 smaller quantities ; when it greatly predominates, as in the pitch- 

 stones, etc., it, of course, gives a vitreous aspect to the rocks ; in all 

 cases it appears to be the uncrystallized residuum of the original fluid 

 or viscid mass ; it does not exhibit double refraction. Wheii examined 

 with a sufficient magnifying power (300 to 600), the microscopic 

 forms are found to be either distinct crystals or rounded granules ; 

 the former are generally acicular prisms, and in the following de- 

 scriptions will be frequently called belonites ; ^ and the entire group 

 of microscopic products will be included under the term microlite, 

 both being convenient terms, employed by Zirkel in his description 

 of the vitreous rocks.'^ 



On the Corriegills shore north of Clauchland Point, a vein of 

 pitchstone from twelve to thirteen feet thick occurs in the face of 

 the cliff not far from the foot pf Dun Fion. At first sight it appears 

 to lie between the beds of Carboniferous sandstone ; it is, however, 

 fortunately exposed for some distance, and a careful examination 

 shows that it cuts gradually, yet distinctly, across the strata, and is 

 therefore clearly intrusive. Another vein of quite similar pitch- 

 stone occurs at a higher level in the same series of sandstones of the 

 Clauchland hills. It is a dark greyish-green vitreous rock with a 

 dull resinous lustre, in which are scattered many small roundish 

 grains of a pale yellow substance, and in some specimens there are a 

 few small crystals of felspar and quartz. 



A thin section, examined under the microscope with a one-inch 

 objective, is seen to consist of an amorphous glassy base, containing 

 numerous long slender prisms of a green pyroxenic mineral; the 

 latter are occasionally isolated, but generally form the axes to which 

 are attached innumerable minute pale green crystals, arranged in 

 exquisitely beautiful groups ; some wonderfully like fronds of ferns, 

 others bearing the closest resemblance both in form and colour to 

 some of the microscopic freshwater algEe ; in fact, the field of view 

 appears to be crowded with minute ferns, and the most elegant 

 sprays and tufts of Batraclwspermiim (Plate I. Fig. 1). The glassy 

 base has a pale yellowish tint in the open spaces between the groups ; 

 but under a higher power the colouring matter is resolved into a 

 mass of translucent granules and minute crystals, the latter being 

 much smaller than the belonites which form the fern-like groups. 

 A comparison of many specimens, affording gradations in size, 

 shows clearly that all these crystalline particles consist of the same 

 pyroxenic mineral as the larger prisms which form the axes. It is 

 worthy of remark that the tufts and aggregations of belonites are 

 invariably surrounded by a border of clear colourless glass (see 

 Fig. 1). Placed between crossed prisms, the section is, if possible, 

 still more beautiful ; the tufts and sprays then appear to be formed 

 of bright gold powder on a perfectly black ground, with fine bril- 

 liant dust scattered in the dark spaces between the groups. The 

 glassy base is a single refracting substance, and therefore dark be- 

 1 Pe\6vr}, "a needle." ' Zeitschr. d. deutsche. geol. Gesel. 1867. 



