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



felspar, converted into felsitic matter, or fragments of quartzite, and 

 fragments of slate, together with grains of magnetite. The fragments 

 of volcanic rock appear to be of an andesitic type. The slate frag- 

 ments are the largest and the most numerous. 



Hartshill Quartzite. 1 

 A very fine-grained pinkish-grey rock resembling quartzite. 

 Under the microscope the rock appears to consist chiefly of 

 irregular crystalline grains of quartz containing numerous minute 

 fluid lacunae, with a few grains of felspar, which from the extinctions 

 are seen in some cases to be microcline, in others anorthite. Grains 

 of a clear, colourless, isotropic mineral are also present, which are 

 probably garnet. Here and there but quite rarely a little micro- 

 crystalline siliceous matter may be seen lying in thin films or occupy- 

 ing small spaces between the quartz grains. The rock is essentially 

 a quartzite. 

 iv., v. and vi. Diorites ; Andesites, or Diorites containing Augite ; 

 Basalt, or Diorites containing Olivine. 2 

 The specimens are taken in geographical order. 

 Marston Jabet. Intrusive in Lower Silurian Shales. 



A rather coarsely crystalline greenish-grey rock, consisting of 

 greyish crystals of felspar, a dark green mineral, calcite and specks 

 of pyrites. 



Under the microscope the section shows, triclinic felspar crystals 

 (labradorite), serpentinous matter pseudomorphous after hornblende, 

 the crystals being frequently about ■£$ inch in diameter and giving 

 in sections transverse to the vertical axis the usual prismatic angle 

 for hornblende. These pseudomorphs are mostly flecked with an 

 opaque yellowish-white substance probably leucoxene. Well-defined 

 pseudomorphs of leucoxene after ilmenite are also present, the altera- 

 tion being so complete that scarcely a trace of the original mineral 

 remains. Spots of pyrites and large patches of calcite occur, and 

 serpentine, frequently fibrous and in spherical groups of divergent 

 fibres, often fills the spaces between the larger crystals. Apatite is 

 also plentiful in well-formed crystals. The rock is a diorite, as 

 pointed out by Mr. Allportinthe paper already cited, where descrip- 

 tions of the microscopic characters of this rock in its fresh condition 

 and in various stages of alteration are very clearly given. 



Griff Farm (South of). Intrusive in Lower Silurian Shales. 



A bluish- to brownish-grey crystalline rock containing specks of 

 pyrites. 



Under the microscope this is seen to be composed of crystals of 

 labradorite and hornblende with a considerable amount of serpentine 

 or a pale greenish substance which appears to be serpentine, and 

 which occasionally occurs in small spherical or fan-like fibrous 



1 See also T. H. "Waller, Geol. Mag. 1866, p. 235. The Quartzite. 



2 See also J. J. H. Teall, Hornblende-Bearing Eocks, Geol. Mag. 1886, p. 351. 



