8 S. All/port — On the Pitchstones of Arran. 



the well-known Scuir of Eigg. The glassy base of this rock is of 

 a velvet-black colour, rather dull lustre, and thickly studded with 

 crystals of sanidine. Under a one-inch objective, a very thin section 

 exhibits a compact grey base full of fine dust, in which there are 

 imbedded crystals and irregular fragments of sanidine, augite, and 

 grains of magnetite. Examined with ^in. objective=520 diameters, 

 the base is seen to consist of an immense number of extremely 

 minute brown prisms, and about an equal number of granules ; many 

 of these are transverse sections of the prisms, others are black and 

 are probably magnetite. These microlites are so crowded in the 

 glassy base as to render it only partially translucent in the thinnest 

 sections ; they may, however, be very well seen along the sides of 

 the felspar and quartz crystals, and in those portions of the base 

 which extend for some distance into the crystals. They are also 

 remarkably well shown in places where the section happens to cut 

 the face of a crystal very obljquely, leaving a wedge-shaped layer 

 of the base extending across it. The average size of the larger 

 microlites is ■g-oV-g- X t-ts-o-o ^f an inch, but the majority of them are 

 much smaller. The sanidine is clear and transparent, and often 

 contains portions of the base, etc. There are also a few crystals of 

 augite and grains of magnetite included in the base. Here and 

 there cavities of an oval form have been filled with a yellow sub- 

 stance, or the centre is yellow with a clear transparent border. These 

 are evidently secondary products, and probably fill spaces previously 

 occupied by some easily decomposed mineral. 



There still remain some interesting rocks connected with the 

 pitchstones, but a description of them must be left for another 

 occasion. 



With respect to the age of the Arran pitchstones, all that can be 

 said at present with certainty is, that they are more recent than the 

 Granite or the Carboniferous sandstones ; they are, however, so inti- 

 mately associated with the basalts and some of the quartz-porphyries 

 — occasionally forming portions of the same dykes — that they are 

 probably of the same age, and may owe their origin to the great 

 Miocene volcanos which seamed the north-western coast with dykes, 

 and poured their huge streams of basaltic lavas over a wide range of 

 country, extending from the North of Ireland through the inner 

 Hebrides to the Faroe Islands and Iceland. 



Since the foregoing observations were written, I have had the 

 pleasure of reading " Geological Sketches of the West-coast of Scot- 

 land," by Professor Zirkel.' Drawn by the hand of a master, these 

 sketches exhibit a more accurate description of the rocks of that 

 interesting district than can be found elsewhere, and are therefore of 

 special interest and importance to English geologists. The paper 

 contains a description of the pitchstones of Arran, and at first sight 

 it occurred to me that my own account of them had thus been 

 rendered unnecessary. There are, however, so many varieties of 

 these rocks, so many points of interest connected with their struc- 

 ' Zeits. d. d. geol. Ges. vol. xxiii. 1871. 



