. The Quarterly journal of the Geological Society of London . lingaugite, glittering in the sunlight, in a dull dark matrix, recalled atonce the characteristic aspect of the Schriesheim picrite. Theboulder had been broken, apparently rather recently, into threepieces, one much smaller than the other two ; and its volume musthave been not much less than a cubic yard. In its weathered sur-face and toughness under the hammer it also resembled the Schries-heim rock. In both, the larger crystals (which are often abouttwo thirds of an inch long) contain a number of dark serpentinous-looking enclosur
. The Quarterly journal of the Geological Society of London . lingaugite, glittering in the sunlight, in a dull dark matrix, recalled atonce the characteristic aspect of the Schriesheim picrite. Theboulder had been broken, apparently rather recently, into threepieces, one much smaller than the other two ; and its volume musthave been not much less than a cubic yard. In its weathered sur-face and toughness under the hammer it also resembled the Schries-heim rock. In both, the larger crystals (which are often abouttwo thirds of an inch long) contain a number of dark serpentinous-looking enclosures, giving to the cleavage-faces an interrupted lustresomewhat resembling (except in the absence of a metallic gleam)that of bastite. The Pen-y-Carnisiog rock looks a little more de-composed ; but macroscopically the resemblance between my twospecimens is so great that one could believe them to have beenbroken from different parts of the same mass. Part of a Slice from a Boulder of Hornblende Picrite near Pen-y-Carnisiog, Anglesey. (Magnified 30 diameters,). a. One of the grains of altered olivine. b, b, b. Aggregated small crystals of hornblende, probably of secondary origin. 138 PROF. T. G. BONNEY ON A BOULDER OF HORNBLENDE When the Pen-y-Carnisiog rock is examined microscopically, thedifference between the two is rather more marked. In it the pre-dominant mineral is undoubtedly hornblende. This occurs underthree forms :—(a) innumerable small acicular or blade-like crystals,in irregular tufted groups, forming a kind of ground-mass: thesevary from a pale green tinge to almost colourless, and are generallyvery feebly, if at all, dichroic; the comparatively small extinction-angle shows them to be hornblende (actinolite) ; and there can beno reasonable doubt that they are of secondary origin; (b) smallcrystals, exhibiting often characteristic cleavages and even externalforms (combinations of ooP and ooPcoo), green-coloured and stronglydichroic; (c) large crystals (those mention
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Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
