560 F. Ridley — Igneous Rocks of the Warwickshire Coal-field. 



streaks approximately parallel to the lamination of the rock. The 

 entire section is profusely spotted with small crystals and fragments 

 of crystals of felspar and augite, the latter mineral being usually 

 represented by pseudomorphs of limonite. Some of the sections of 

 augite crystals undergo maximum extinction at an angle of about 

 39° from the vertical axis, while other sections transverse to the 

 vertical axis show approximately right angles formed by the adjacent 

 faces of the oblique rhombic prism. The felspars are generally 

 labradorite. Taking into consideration all the characters presented 

 by this rock, it seems highly probable that it is an altered andesitic 

 tuff. 



"Blue Hole," east of Caldecote Windmill. 1 Caldecote Series. 

 (The Quartz- Porphyry.) 



Eock composed of pink and greyish-white crystals of triclinic 

 felspar with grains of quartz, in a dark bluish- or brownish-grey 

 matrix, the latter constituting the smaller proportion of the rock. 



Under the microscope the general appearance is that of a tuff, 

 composed of crystals or fragments of crystals of triclinic felspar and 

 quartz more or less corroded, together with fragments of eruptive 

 rock of an andesitic type, containing serpentinous pseudomorphs 

 after pyroxene. The porphyritic felspar crystals, all of which have 

 their angles rounded, are often corroded superficially and their angles 

 of extinction indicate, as a rule, that they are labradorite. The 

 quartz crystals are also rounded, and frequently show inlets of corro- 

 sion, which are much more strongly marked than in the felspars. 

 The material in which these crystals and fragments are embedded is 

 felsitic in character, and as it seems to be continuous with the felsitic 

 matter which fills the corrosion-creeks in the quartz-crystals, it 

 appears probable that it may be a product of devitrification, and that 

 the rock is a tuff resulting from fragments taken up in a magma 

 which, on solidification, assumed a vitreous condition. The general 

 appearance of this rock under the microscope is rather like that of 

 certain porphyritic pitchstones occurring in Saxony, except that in 

 the latter the porphyritic crystals or fragments are usually less 

 numerous and the ground-mass retains its glassy character. It seems 

 probable that this rock from Caldecote is part of a lava-flow or dyke 

 which has taken up fragments of other rocks in such quantity that 

 it simulates an ordinary volcanic tuff. 



"Blue Hole," east of Caldecote Windmill. Caldecote Series. 

 (Junction of Quartz-Porphyry and Andesite). 



Compact bluish-grey to brownish-grey rock with splintery fracture. 

 The small specimen contains in one part greyish-white to pinkish- 

 white specks of felspar, the largest being about iV inch in diameter. 

 Minute grains of quartz are also visible with a lens. 



Under the microscope, between crossed Nicols, the section seems 

 to consist chiefly of a crypto-crystalline to micro-crystalline ad- 

 mixture of felspar and quartz, with some irregularly- shaped and 

 usually rounded grains which, by ordinary transmitted light, appear 

 of a brownish-green colour, and which are probably altered fragments 

 1 See also T. H. Waller, Geol. Mag. 1886, p. 323. 



