428 PEOF. J. W. JUDD ON" THE VOLCANIC ROCKS 



Studied microscopically (see Plate XIII. fig. 8), the rock is found 

 to be one of remarkable interest and beauty. Some portions of the 

 glassy base are seen to be quite as free from alteration as any Ter- 

 tiary or Recent obsidian. Other portions exhibit every stage of the 

 process of secondary change, whereby it passes into the white decom- 

 position-product to be hereafter described. 



The minerals of the first coDsolidation in the rock consist of 

 numerous large crystals of felspar and a few scattered individuals of 

 biotite. 



The porphyritic felspar-crystals, which are sometimes of very 

 considerable size, are usually perfectly fresh and unaltered. They 

 in all cases belong to triclinic species, and usually give extinctions 

 characteristic of oligoclase. Their angles are usually rounded, and 

 they sometimes show evident marks of corrosion by the action on 

 them of the magma in which they are enclosed. Sometimes they 

 are bent and cracked, and in these cases the development of the 

 twin lamellse has been clearly determined by the strains to which 

 crystals have been subjected. Not unfrequently, crystals are found 

 broken into fragments, and these fragments can be recognized lying 

 disunited in the midst of the glass ; in one section I observed a 

 single crystal of felspar which had been broken into no less than 

 nine fragments, the fractured edges of which corresponded perfectly, 

 although separated by the glassy mass in which they lay. 



The biotite is of a deep brown tint, highly pleochroic, the 

 absorption along the C axis being so strong that the crystals, in 

 certain positions, appear absolutely black and opaque when rotated 

 over the polarizer ; in other positions they give various rich shades 

 of brown. The biotite-crystals are often bent and frayed out along 

 the principal cleavage-planes ; not unfrequently they show the black 

 margin so common in the biotites and hornblendes of andesites. 

 Traces of alteration are seen in some of these biotite-crystals, the 

 mineral sometimes appearing to pass into the dark blue and strongly 

 pleochroic chloritoid *. 



The minerals of the second consolidation consist of imperfectly 

 developed microlites of felspar, many of them exhibiting step-like 

 terminations. In no case have I been able to distinguish twin- 

 lam ellse in these crystals; they are either untwinned or simply 

 twinned on the Carlsbad type. There thus appears to be every 

 ground for regarding these minute crystals of the second period of 

 consolidation as being orthoclase. 



The glassy base contains numerous trichites, often forming 

 beautiful stellar groups, with dark-coloured globulites ; the dis- 

 position of these and the felspar microlites of the second consolida- 

 tion with respect to the large porphyritic crystals reveals a most 



* Chloritoid, the optical characters of which have been so well studied in 

 recent years by Barrois (Bull. Soc. Min. Fr. vol. vii. 1884, pp. 37-43), Von 

 Lasaiilx (Sitzungsber. d. niederrhein. Ges. in Bonn, 3 Dec. 1883), and Lacroix 

 (Bull. Soc. Min. Fr. vol. ix. 1886, p. 8), is by no means rare among the Scottish 

 rocks. I find it to occur very abundantly in the interesting rock of Ailsa 

 Craig, for opportunities of studying which I am indebted to Mr. Blackwood of 

 Kilmarnock 



