210 PROF. BOSWELL OiN r THE CRETACEOUS AND [vol. lxxix, 



Staurolite is frequently met with, and in some samples is 

 extremely abundant. The grains are often large (02 mm. to 

 0"3 mm. in diameter) and irregular in shape, but never ragged 

 like many of those described from the neighbouring Trias. 1 They 

 are delicately pleochroic in golden yellows, and usually show the 

 emergence of a single optic axis. 



When topaz occurs (as in certain samples from Smallacombe 

 Goyle), it is often plentiful, and the grains are of large size, 

 irregular form, clear and glassy. When the shape of the grains is 

 determined by the basal cleavage on which the grains lie, they 

 yield a biaxial directions-image, the emergences of the optic axes 

 being usually on the edges of the field, owing to the wide optic 

 axial angle. Frequently, however, the grains do not lie on the 

 basal plane, but are chips or wedge-shaped fragments approximating 

 to that form (see fig. 3, p. 214). Then they yield a partial figure, 

 but not the emergence of a single optic axis. The high refractive 

 index, low birefringence, and positive sign serve to confirm the 

 identification. In certain cases the grains are full of dusky 

 inclusions. 



The clear and non-pleochroic andalusite bears some resem- 

 blance at first sight to topaz, but it may be distinguished by the 

 negative sign, the greater fraying of its edges, the inclusions, etc. 

 Although its average refractive index is higher than that of topaz, 

 more light rays are reflected from the edges of the topaz-grains, 

 because these edges are almost perpendicular to the glass slip. 

 The fainter borders of the andalusite can be shown by focussing 

 to be due to the wedge-shaped edge of the grains. 



Of the iron-ores, magnetite is absent, and ilmenite is, on 

 the whole, rare. The latter mineral is either black and lustrous, 

 or altered to limonite and leucoxene. 



Brookite occurs in several samples. The grains are small and 

 flat, and not bounded by crystal-faces. The colour is from brownish 

 to that of straw, but the characteristic basal striatum is frequently 

 absent. The grains display excellently the typical dispersion ; they 

 do not extinguish completely, and are uniaxial for green light. 



Corundum is a rare constituent. When present it is recog- 

 nized by its relatively high refractive index, its form (usually flat 

 and hexagonal in grains resting on the basal pinacoid), and its 

 patchy ' royal-blue ' colour. In prismatic grains it is pleochroic. 



Particoloured red and yellow cas site rite has also been 

 observed. 



Garnet has been noted, but very seldom, and then as solitary 

 grains, which are generally colourless or faintly pink. They 

 are either isotropic, or, rarely, are anomalously birefringent. 



Chert in grains showing aggregate polarization of chalcedonic 

 type is not uncommon. 



Amonsf those minerals that are abundant in the rocks of Devon 



1 H. H. Thomas, Q. J. G. S. vol. lxv (1909) p. 234. 



