﻿78 



MB. H. G, SMITH ON THE 



[vol. lxxii, 



usual character, almost absent in basal sections, and strong in those 

 sections that show cleavage, the maximum absorption taking place 

 when vibrations are parallel to the cleavage. Polarization -colours 

 are fairly bright pinks and greens, and extinction is straight. 

 Basal sections give a pseudo-uniaxial figure, the optic axial plane 

 coinciding with the plane of symmetiy. 



Small felspar-crystals form the bulk of the rock ; they are, how- 

 ever, almost invariably decomposed. They are rectangular, the 

 length being about three times the breadth ; they are altered more 

 completely near the centre, the margins being comparatively trans- 

 parent ; they do not clear up when treated with acid. The polari- 

 zation-colours are the usual first-order greys and yellows. Lamellar 

 twinning is common, but there are cases where twinning is absent. 

 The lamellse usually extinguish at a fairly high angle, and, as a 

 rule, do not extend to the periphery of the crystal. The outer 

 zone does not show twinning, and extinguishes either in the straight 

 position or with a very small angle. The felspars present are 

 considered to be orthoclase and a plagioclase which is an acid 

 labradorite or andesine. 



Small apatite-crystals are fairly abundant. They are recognized 

 by their hexagonal cross-sections, the basal fracture in prismatic 

 sections (which are commonly much elongated), the high refractive 

 index, grey polarization, straight extinction, and negative sign of 

 elongation. 



Small magnetite- and pyrite-crystals are common. 



Forming a considerable proportion of the rock are large pseudo- 

 morphs (preserved in a mixture of rhombohedral carbonate, quartz, 

 and iron oxides) which have the characteristic form of olivine, and 

 are certainly replacements of that mineral. It is these pseudo- 

 morphs that show as pinkish areas in hand-specimens. Other 

 accumulations of rhombohedral carbonate are considered by 

 Dr. H. H. Thomas to be pseudomorphous after augite. 



A green chlorite is found in interstitial areas bounded by the 

 felspars ; it has replaced some ground-mass which in no part of 

 the rock is seen in the original condition. Less frequently 

 the replacement has been effected by quartz. The interstitial 

 chlorite sometimes encloses well-formed rhombs of carbonate, which, 

 in a few cases, show zoning ; and occasionally, projecting into the 

 chlorite, rosettes of radiating quartz-crystals are observed. 



These minerals, quartz, chlorite, and rhombohedral carbonate, 

 together with idiomorphic orthoclase, in a few cases occur in 

 approximately circular patches (PI. VIII, fig. 2) which are 

 bounded by biotite-crystals arranged with their length tangential 

 to the circumference of the circle. The minerals of these patches 

 are probably the result of the infilling of cavities which originated 

 after the crystallization of the biotite. 1 The felspar-crystals project- 

 ing into these areas, and also those projecting into the interstitial 



1 See Sir Jethro Teall, 1 The Amygdaloids of the Tynemouth Dyke ' Geol. 

 Mag. dec. 3, vol. vi (1889) pp. 481-83. 



