S. Allport — On the Igneous Rocks of Arran. 541 



spherical concretions, set in a compact matrix. The spheres have a 

 distinct fibrous radial structure, and are frequently coated with a 

 white film. In a very thin section, in ordinary light, the spheres 

 exhibit a well-defined circle, bounded by a line of minute grains of 

 iron oxide, but the fibrous structure is not so distinct ; in fact, it then 

 appears to be simply a radial arrangement of the particles of a fine 

 dust scattered through a dull uniform base ; dark greenish aggrega- 

 tions of this substance sometimes form an irregular nucleus, throw- 

 ing ofi" rays towards the circumference ; frequently, however, the 

 centre is free from them, and there is then no appearance of any 

 sort of structure. These green patches also occur in the matrix, and 

 both spheres and matrix appear to be composed of precisely the same 

 substance. Placed between crossed Nicols the appearance is com- 

 pletely changed, and it is at once seen that the matrix has a felsitic 

 structure, and that some of the spheres are also composed of portions 

 of the same substance, which have, however, undergone a process 

 of aggregation and radial arrangement in globular masses ; but the 

 felsitic structure is still quite as evident as in the base. Many of 

 the spheres are, however, composed of two or more concentric 

 layers ; in some there is a felsitic nucleus surrounded by radiating 

 groups of the green dust ; in others the nucleus consists of grains of 

 quartz only. The globular concretions here described diifer entirely 

 in structure and appearance from the spherolites occurring in the 

 pitchstones, and it is quite evident that the rock is a felsite. Some 

 specimens may, at first sight, be readily taken for a dull perlitic 

 pitchstone, but a microscopic examination shows very clearly that it 

 is not a pitchstone, and that the structure is not perlitic. 



On the shore south of Tormore there is a compound dyke about 

 20 feet wide, the two sides consist of basalt, while the central part, 

 12 feet in width, is composed of a hard light grey felsite, containing 

 crystals of quartz and a few of felspar ; the latter are chiefly ortho- 

 clase, but plagioclase is also present. The base is felsitic, and contains 

 many perfectly clear spherolites with radiating belonites ; this is the 

 only indication of structure in ordinary light, but between crossed 

 Nicols there is a well-defined black cross. The larger quartz crystals 

 are surrounded by a continuous clear band, containing tufts of ra- 

 diating belonites, and between crossed Nicols the tufts are seen to 

 mark the places of imperfectly formed spherolites, more or less 

 complete Ijlack crosses being then visible. Many belonites, and 

 small opaque grains, are scattered through the base, and they assume 

 a stream-like arrangement round the sides of the larger crystals. 



In Monamore Glen, about a mile above the mill, there is a small 

 quarry worked for road metal. The rock is a hard felsite, varying 

 in colour from greenish-grey to light brown ; it contains a little 

 felspar, and numerous crystals of quartz ; the latter being surrounded 

 by a green border, give the rock an unusual appearance. The base 

 is felsitic, contains many green belonites, and is quite as distinctly 

 spherolitic as the specimen just described. In addition to many 

 clear spherolites, there are others of a dull green substance, having 

 a radial arrangement, but not distinctly crystallized. The quartz 



