232 ME. H. H. THOMAS ON THE [A-Ug. I909, 



intergrowth as figured by Thiirach, l Cayeux, 2 and others (PL XII, 

 fig. 1, a & c). Occasionally, the basal plane is absent, and its place 

 is taken by low pyramids of some form such as {hhl}, as in the 

 specimen from the Lower Breccias of the Ness at Teignmouth 

 (PL XII, fig. 1 b). 



(b) The pyramidal variety occurs in steel-blue or yellow crystals 

 with the basal plane small or absent, and having well-developed 

 pyramid-planes modified only by narrow prisms {110}. The 

 pyramid-planes are deeply striated parallel to their intersection 

 with the prism (PL XII, figs. 3 & 5). These forms are accurately 

 described by Thiirach, who, in addition, mentions microscopic 

 anatase with a pyramid of the second order {hoi} and a double 

 pyramid {hkl}. These I have not noticed on any crystals from 

 the New Red rocks. 



Anatase belongs essentially to the crystalline metamorphic rocks; 

 but it may be formed from a variety of titaniferous minerals on 

 their decomposition, as, for instance, from ilmenite, sphene, etc. 3 

 Mr. J. B. Scrivenor described the alteration of ilmenite into anatase 

 (PL XII, fig. 7) in the Triassic sandstones of Cheshire, 4 and stated 

 that in his opinion the change had taken place since the deposition 

 of the sandstone. In the south-western area I have observed many 

 crystals of anatase attached to ilmenite or leucoxene ; and there is 

 little doubt, from the sharpness of the angles and the general 

 fragility of the crystal-groups, that nearly all the tabular anatase 

 has been formed as Mr. Scrivenor suggested. 



Some of the pyramidal and more acute forms, however, show 

 unmistakable signs of wear ; their edges are rounded, and the 

 crystals themselves far from perfect. These most certainly have 

 been derived from pre-existing rocks. 



Calcite. — Calcite enters into the composition of most of the 

 New Bed rocks in the neighbourhood of calcareous inliers of older 

 rocks, and in some divisions of the Bed rocks far removed from 

 such calcareous masses. It occurs chiefly as a calcareous cement, 

 but most probably also as detritus. Some of the upper marls are 

 remarkable for their high percentages of calcium-carbonate when 

 quite remote from any older calcareous rocks. 



Haematite. — This mineral also occurs as a cement, and is often 

 associated with secondary growths of quartz. It exists as thin 

 plates coloured a deep red ; also as minute botryoidal grains, made 

 up of spheroidal masses with radiate structure which give a black 

 cross between crossed nicols. It is evidently of secondary origin. 



1 'Ueber das Vorkommen mikroskopischer Zirkone & Titan-Mineralien in 

 den Gesteinen ' Wiirzburg, 1884. 



2 Mem. Soc. Geol. du Nord, vol. iv, no. 2 (1897) pi. x. " 



3 A. Lacroix, ' Sur qq. Cas de Production d'Anatase par Voie Secondaire ' 

 Bull. Soc. Fran<j. Min. vol. xxiv (1901) p. 425. See also W. Prinz, 'Les 

 Oxydes de Titane . . . . de qq. Roches du Brabant ' Bull. Soc. beige Geol. 

 vol. xxi (1907) Mem. p. 383. 



4 Min. Mag. vol. xiii (1903) p. 348. 



