specimen from a point on the Coquet, half-a-mile above Shillmoor Farm, 

 was analysed by Mr. WALLER and yielded the following result : 



Si0 2 ... 67-9 



A1A ... 15-7 



FeA ... 8-0 



CaO ... 1-4 



MgO ... 1-5 



Na 4 O ... 1-5 



K 2 ... 5-6 



Loss ... 8-7 



100-3 



Scotland. Rocks of the group now under consideration abound in 

 Scotland, but with the exception of those which occur in the island 

 of Arran they have not as yet received much attention at the hands 

 of modern petrographers. Intrusive felsites and quartz-porphyries are 

 associated with the principal granite masses and form innumerable 

 dykes in certain districts, especially in the Southern Uplands. Felsitic 

 lavas and tuffs occur associated with the Old Red Sandstone and to 

 a certain extent also with the Carboniferous rocks of the Midland 

 Valley. They are again found in the Western Isles, where they 

 belong to a much later geological period. 



The microscopic characters of the felsitic and vitreous rocks of 

 the Isle of Arran have been described by Professor ZIRKEL^ and 

 Mr. ALLPORT. ( 2 ) The pitchstones of Arran are also referred to by 



VOGELSANG/ 3 ) 



The felsitic rocks have at various times received such names as 

 hornstone, claystone, claystone-porphyry, quartz-porphyry, felspar- 

 porphyry, quartz-felsite and spherulitic felsite. The vitreous rocks have 

 a resinous lustre and have always been known as pitchstone. Both 

 varieties occur as intrusive sheets, dykes and more or less amorphous 

 masses. They are of later date than the Carboniferous rocks of the 

 Island. < 4 > In all probability they belong to the same period as the 

 granites, granophyres and felsophyres of Mull and Skye. 



The pitchstones of Arran are generally of a dark bottlo-green 

 colour. Some varieties are almost black. Porphyritic crystals of felspar 

 and quartz may or may not occur. Globular spots of felsite occur in 

 certain varieties. These may or may not show a radial structure. 



Under the microscope the principal constituent is seen to be a 

 nearly colourless isotropic glass, more or less charged with greenish 

 acicular microlites. These microlites vary considerably in size. Some- 

 times they are so minute and so numerous as to give merely a dusty 

 aspect to the section when viewed with a low power ; at other times 



(1) Z.D.G.G. (1871), p. l. 



(2) G.M., 1872, pp. 1-10 and 536-545 ; also G.M., 1881, p. 438. 



(3) Die Krystalliten, p. 124. 



(4) Professor ZIEKEL describes the pitchstone -sheet on the Corriegills shore as iuterbedded 

 with the sandstones. Mr. ALLPOKT has shown that this is not so. It is in reality intrusive. 



