Hcv. T:G. Bonney — Pitchstones and Felsites of Arran. 505 
Reviewing tlieu the above facts, we seem justified in asserting 
that in some of the pitchstones there is the following succession of 
phenoinena — 1. The formation of the larger porphyritic crystals ; 
2. The dififerentiation of the matrix, which produces the streaky or 
bauded structure ; 3. The formation of the belonites ; 4. The aggre- 
gatiou 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. Witli 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 brauching 
dyke of basalt (see Fig. 2). The former felsite is very flaggy, with 
Fig. 2. — Dykes near Drumadoon Point. 
A. Porphyritic Felsite. B. Basalt. C. Compact Felsite. 
C'. Spherulitic 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 witliin 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 microlitbic structure. Can it be 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 found in some of the coarser leucite 
lavas of Yesuvius. 
