S. Allport — On the Igneous Mocks of Arran. 539 



2. A brown pitclistone, of duller aspect than No. 1, containing 

 spheroidal nodules of light brown compact " hornstone," having a 

 bluish-grey nucleus ; the nodules are from 1 to 2 inches in diameter, 

 and contain many crystals and grains of quartz, with some of felspar. 



3. A specimen, containing the two former varieties, with a little 

 red " hornstone." 



4. A compact red "hornstone," with crystals of quartz and fel- 

 spar ; a portion of one specimen also exhibits a minute spherolitic 

 structure. 



Under the microscope a section of No. 1 exhibits, in ordinary 

 light, a pale yellow glass, full of well-defined circular aggregations 

 of a reddish-yellow substance, which appears indistinctly granular ; 

 these are the grains seen in the specimen ; they will be described as 

 spherolites. Great numbers of short green belonites are scattered 

 through the base, and there are many crystals of quartz, orthoclas© 

 and plagioclase, the latter are beautifully striated. A crystal of 

 quartz or felspar sometimes forms the nucleus of a spherolite, 

 numbers of belonites radiate from it, and the group is surrounded 

 by one or more concentric coloured bands ; frequently, however, the 

 spherolites have no distinct nucleus. The true character of the rock 

 is better seen in polarized light ; between crossed Nicols the glassy 

 base of course remains dark, and on this black ground numerous 

 spherolites and small patches of felsitic matter are very w^ll seen. 

 The spherolites invariably exhibit double refraction, but no felsitic 

 structure; in ordinary light many are quite clear, and the only 

 indication of a radial structure is the direction in which the belonites 

 lie. Between crossed Nicols, however, a black cross is seen, and on 

 rotating either of the prisms the arms of the cross also rotate ; this 

 is of course indicative of a minute radial crystallization. 



The dull brown pitchstone. No. 2, which contains the nodules, 

 exhibits many spherolites like No. 1, aggregations of felsite, and 

 also characteristic groups of belonites. 



The " hornstone " nodules consist of the usual glassy base, nearly 

 filled with a fine brown dust, which renders it dull and semi-opaque ; 

 a very thin section exhibits however a somewhat similar structure 

 to No. 2 ; but the belonites are longer, and there are very few fern- 

 like groups ; larger portions of the base are felsitic, quartz crystals 

 are more abundant, and there are several small masses of amorphous 

 quartz. 



The grey nucleus of the nodules exhibits a characteristic felsitic 

 texture, together with imbedded crystals of quartz and felspar. 



The red "hornstone," No. 4, consists of a very compact felsitic 

 base, with much colouring matter disseminated through it, and it 

 also contains crystallized quartz and felspar. 



I am fortunate in being able to give the results of a very careful 

 analysis of two of these rocks by Mr. J. Arthur Phillips, who kindly 

 undertook the examination, and made a double analysis of each. 

 No. I. is the " hornstone " nodule. IL, the red felsite or "hornstone." 

 III., a globular felsite, to be presently described ; also analyzed by 

 Mr. Phillips :— 



