F. Ridley — Igneous Rocks of the Warwickshire Coal-field. 561 



of crystals of pyroxene or arnphibole. A similar green or brownish- 

 green substance (chlorite ?), accompanied by minute specks of opaque- 

 white matter (kaolin), constitutes irregular stringy markings and 

 sinuous bands, which traverse the section, in some cases simulating 

 fluxion structure, the material between such bands sometimes differ- 

 ing in texture from that on either side. This seems to be due to the 

 presence of fragments of rock, whose margins are ill-defined, except 

 through the contact of the chloritic films and strings. Small angular 

 and rounded fragments of felspar crystals are plentifully distributed 

 throughout the section, and here and there opaque grains of ilmenite, 

 partly converted into leucoxene, are visible. 



The porphyritic crystals and fragments of crystals of felspar 

 mostly give extinction angles approximating to 4:0°, and may therefore 

 be regarded as anorthite. In a few cases, however, labradorite 

 occurs. 



Judging from the section, it seems probable that this rock is a 

 tuff composed of lapilli of some rocks allied to andesite or porphyrite 

 in which the ground-mass was once vitreous or partly so. 



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

 (Andesite.) 



Compact purplish-grey rock, in which a few minute felspar crystals 

 may be detected with a lens. 



Under the microscope the section is seen to consist of crystals of 

 triclinic felspars and magnetite in a felsitic or aphanitic ground- 

 mass, in which numerous very minute crystals occur, which are 

 apparently hornblende, since in some cases they give an extinction 

 angle of 19°. 



If these small crystals be hornblende, the rock is probably a horn- 

 blende andesite. It is somewhat decomposed, the specimen effer- 

 vescing slightly when treated with acid. Isotropic matter seems to 

 be present and it is quite possible that the rock may have originally 

 possessed a more or less glassy ground-mass, some of the unaltered 

 vitreous matter being apparently still present. 



in. Hartshill Quartzite with Breccia at its base. 



Base of Hartshill quartzite, half a mile S.E. of Hartshill. 



A coarse breccia consisting of angular fragments, embedded in a 

 bluish-grey to purplish matrix. On a cut surface of the specimen 

 some of the fragments are of a dark slate-colour or purple, while 

 others are of a rather pale greenish-grey. The fragments do not 

 seem to exceed half an inch in diameter, and most of them are of 

 much smaller dimensions. 



Under the microscope the fragments are seen to be of a mixed 

 character. A few consist of eruptive rock, one or two showing distinct 

 felspar crystals (labradorite), while one fragment contains small 

 vesicles filled with a pale green substance (chlorite?). Quartz 

 grains constitute a large proportion of the rock, and most of the 

 other fragments and grains consist either of greatly decomposed 

 1 See also T. H. Waller, Geol. Mag. 1886, p. 324. The Dark Basic Rock. 



DECADE III. VOL. Ill MO. XII. 36 



