154 Alfred Brammall- — Intrusive Rock, 



treatment with, dilute hydrochloric acid, owing to the presence of 

 calcite in the rock-mass. 



Another variety, occurring as a selvage in contact with the shale, 

 is a dark-grey compact rock, often banded, but otherwise of basaltic 

 aspect, though no crystals are visible even on examination with the 

 lens. The material forming the veins and strings intrusive in the 

 shales is an earthy substance with the appearance, texture, and 

 ' feel ' of hardening putty. 



4. Detailed Penological Description. 



The felspar is mainly plagioclase, which forms slender idiomorphic 

 prisms up to -fe inch in length, usually less, showing between 

 crossed nicols broad but inconstant twin laminse of the albite type, 

 occasionally combined with Carlsbad and pericline twinning ; an 

 extinction angle of 27°-29° points to labradorite of basic composition. 

 The well-individualized albite laminse of the middle of the felspar 

 prisms are commonly flanked by a narrow marginal band, the 

 extinction axis of which appears to traverse the band from side to 

 side as the stage is rotated — an optical feature due to zonal variation 

 in chemical composition. Orthoclase is a very subordinate constituent 

 of a scanty granular ground-mass. 



The felspar has a pronounced yellow-brown tint due to iron 

 staining, and has undergone considerable alteration, resulting in the 

 development of granular calcite and epidote," aggregated mainly in 

 the interior. The alteration has only slightly obscured the definition 

 of the albite laminae — the contour of the crystal not at all. 



The prisms form an open framework, and are frequently inter- 

 penetrant at the nodes, where they tend to group themselves radially 

 after the manner of the glomeroporphyritic aggregates common in 

 intrusive and volcanic rocks. This framework, together with hoi'n- 

 blende, calcite (in plates or granules), a little granular orthoclase, 

 and rather conspicuous grains of magnetite, is embedded in a nearly 

 colourless or pale-green serpentine associated with subordinate patches 

 of a greenish dichroic substance referred to chlorite. 



In the normal type of the rock the felspar prisms may be enclosed 

 in hornblende ; in some varieties the two minerals interlock along 

 a sinuous or serrated line, a portion at least of the hornblende having 

 been fluid at a stage when the crystallization of the felspar was 

 incomplete. In yet another variety the hornblende is wholly 

 idiomorphic and independent of the felspar. 



Hornblende is very abundant as a dark-brown variety which, 

 when unaltered, is quite transparent and shows intense brown 

 absorption. Often, however, it is so dark as to be almost opaque, 

 suggesting a somewhat advanced stage of alteration. The mineral is 

 conspicuously idiomorphic, giving the usual lozenge-shaped basal 

 sections (acute angles truncated by the clinopinacoid traces) and 

 long prismatic sections. Twinning on the orthopinacoid is common. 

 Hornblende may enclose plagioclase prisms ophitically, or may be 

 conjugate with felspar along a straight, curved, or serrated junction 

 line, or may be an entirely independent constituent. All stages of 

 alteration to serpentine are observable ; the serpentine either merely 



