﻿Rev. T. G. Bonney — Pitchstones and Felsites of Arran. 505 



Eeviewing then the above facts, we seem justified in asserting 

 that in some of the pitchstones there is the following succession of 

 phenomena — 1. The formation of the larger porphyritic crystals ; 

 2. The differentiation of the matrix, which produces the streaky or 

 banded structure ; 3. The formation of the belonites ; 4. The aggre- 

 gation of them into the alga-like clusters; 5. The formation of 

 spherules. Of these, 1 and 2 as a rule take place before the motion 

 of the rock as a mass is quite arrested, though probably it has at- 

 tained a pasty condition, and — at least in 2 — the position of its mole- 

 cules in space is not greatly altered subsequently. With regard to 

 3 and 4, the evidence is more conflicting ; the belonites sometimes 

 giving evidence of subsequent pressure or strain, sometimes of 

 having formed in a mass in equilibrium ; 5 has occurred — and in con- 

 nexion with it the perlitic structure — when all motion except mole- 

 cular has become impossible. 



Passing now to the felsites, we find an interesting case on the 

 shore north of Drumadoon ; there a dyke of compact felsite is 

 divided from another mass of porphyritic felsite by a branching 

 dyke of basalt (see Fig. 2). The former felsite is very flaggy, with 



Fig. 2. — Dykes near Driimadoou Point. 



A. Porphyritic Felsite. B. Basalt. C. Compact Felsite. 

 C Splierulitic Felsite. 



some slight appearance of a generally minute spherulitic structure, 

 but the character of the rock changes for the last half-yard or so 

 adjacent to the basalt ; here the flaggy structure is lost, the rock 

 becoming massive, of porcellanous aspect, with well-defined spheru- 

 litic structure. On examining a surface of this carefully, we see a 

 polygonal network, indicated often by darker lines, and within each, 

 reticulation one or more well-defined spherules. Under the micro- 

 scope (Fig. 3) we see the network indicated clearly by darkish 

 lines ; sometimes faint, but oftener well-defined, like a string over- 

 grown with dusty mould. The inner circles are marked with fainter 

 lines of the same. On crossing the prisms, we see the structure 



tion of the larger crystals of felspar and some other minerals seems anterior to the 

 setting up of microlithic structure. Can it he that in many cases these have been 

 formed in some prior cooling of the rock, and that the last elevation of temperature 

 previous to the ejection of the rock only fused the ground-mass ? Except on some such 

 theory it is very hard to explain such cases as are foimd in some of the coarser leucite 

 lavas of Vesuvius. 



