S. Allport — On the Igneous Rocks of Arran. 543 



The felsites just described all form dykes or veins in the sand- 

 stones ; but there is a rock, of similar appearance, which occupies 

 a considerable area in the vicinity of Lagg. It nowhere reaches the 

 coast, but is first seen a few hundred yards south of Clauchog farm, 

 and extends in a northerly direction for at least two miles ; and 

 from Sliddery Water, on the west, to the Cloined burn, near the 

 well-known shell-bed, on the east, a distance of a mile and a half ; 

 how much further it may extend in both directions is uncertain, 

 as I had no opportunity of tracing it. In Sliddery Water it is first 

 seen in the bed of the river, about 700 yards above the bridge, where 

 there is a fine junction with the sandstones. The side of the felsite 

 is vertical, with the ends of the sandstone strata abutting against it, 

 and dipping away at an angle of 30° ; the felsite here crosses the 

 river, and occupies both banks as far as Glenrie Mill. It appears to 

 be continuous within the limits observed, and in the Cloined burn 

 the relations of the two rocks are the same as in Sliddery Water. 

 In this instance a mass has probably flowed over the sandstone, and 

 has been brought into its present position by faults. 



The rock is very uniform in character, generally of a light-grey 

 colour, but often marked with reddish-brown bands and concentric 

 rings. The base is finely granular, and contains many small crystals 

 of felspar and quartz, with numerous grains of magnetite. Under 

 the microscope the base appears distinctly granular, and contains 

 much quartz ; orthoclase and plagioclase are both present, though 

 much altered ; the quartz is well crystallized, sometimes exhibiting 

 short prisms doubly terminated. 



Porphyritic Felsites, or quartz porphyries, as they have generally 

 been called, occur in many places in Arran, frequently as dykes in 

 the Carboniferous sandstone, or forming extensive overlying masses. 

 The best localities on the west coast are Drumadoon Point, Leac a 

 breac between Blackwater foot and Sliddery Water, and Benan 

 Head ; and on the east coast. Dun Dhu between Brodick and Lamlash. 

 As Zirkel has given an account of these rocks in the paper cited 

 above, a brief description of two varieties will now suffice. Benan 

 Head is formed entirely of a mass of various intrusive rocks, of 

 which a beautiful " quartz porphyry " forms the greater part. It is 

 a pale-grey rock, containing numerous large crystals of orthoclase 

 and quartz in a compact base ; the orthoclase is clear and glassy, like 

 sanidine ; the quartz is well crystallized in short prisms doubly 

 terminated. 



Under the microscope, in polarized light, the base exhibits a 

 perfectly characteristic felsitic structure, with a very few imperfectly 

 formed felspar crystals, and a little quartz ; there are also many 

 small black grains of magnetite. Nearly all the felspar is orthoclase, 

 but Zirkel's statement that there is no trace of plagioclase requires 

 to be modified, as one of my sections contains a crystal which 

 exhibits well-defined coloured striae. The larger crystals of quartz 

 and felspar contain many cavities, with included portions of the base. 



The columnar " porphyry " at Drumadoon Point is of a darker 

 colour than the preceding rock, and some of the orthoclase crystals 



