Notices of Memoirs — Geology of the Forest of Dean. 23 



Magnetite (titanomagnetite) is present in irregular, often rounded 

 grains, but some crystals show isometric sections. A highly refracting 

 border (leucoxene ?) surrounds some of the grains, indicating the 

 presence of titanium. Scott Elliot noted the occurrence " in the 

 hills behind Sierra Leone" of a titaniferous iron-ore yielding 52 per 

 cent of metallic iron and 14 per cent of Ti Og. Giirich, too, saw in 

 a private collection of minerals in Freetown lumps of magnetite 

 as large as the fist. I venture to conclude that this ore-body occurs 

 as a segregation within the norite ; indeed, the recognition of the 

 petrographic character of the rock would lead one to anticipate the 

 existence of such segregations. These indications ought to be 

 followed up, as they might lead to the discovery of important ore- 

 deposits. The matter is one for the attention of the Imperial 

 Mineral Resources Bureau. 



Estimates of the percentage composition of these rocks yielded the 

 following results in the case of (A), a specimen of what I have called 

 the average rock, and (B) the aplitic facies : — 



Plagioclase . 

 Olivine . 

 Diallage ~\ 

 HyperstheneJ 



Magnetite 



The order in wliich the minerals crystallized is as follows : 

 labradorite, olivine, diallage, hypersthene ; magnetite uncertain. In 

 brief, the fine-grained rock is a melanocratic norite (rag - micro- 

 norite), while the coarser varieties are leucocratic oliviue-norites 

 (Ig — to Ig- subnorite). I invite attention here to a change which 

 ought to be made in the subdivision of the gabbroitic rocks ; namely, 

 that in deciding whether a rock is to be attached to gabbro or to 

 norite the olivijie should he reckoned along loith the rhombic pyroxenes, 

 since the latter minerals arise by the addition of silica to olivine. 

 Thus all troctolites and " olivine-gabbros " (including those above 

 described) in which the sum of olivine plus rhombic pj'roxenes 

 exceeds that of the monoclinic pyroxenes should be regarded as 

 norite (subnorite), not as gabbro. 



ISrOTIOIES OIF" ]yEE!DVEOIK,S. 



On the Geological Stexjctuke of the Fokest of Dean.' By 

 T. Franklin Sibly, D.Sc, F.G.S., Professor of Geology in tlie 

 University College of South Wales and Monmouthshire, Cardiff. 

 (A paper read before the Forest of Dean Branch of the National 

 Association of Colliery Managers on October 25, 1917.) 



IN 1894 Dr. Pt. Kidston correlated the Coal Measures of the Forest 

 of Dean with the true Upper Coal Measures.^ In 1910 Dr. T. T. 

 Groom, reasoning from this correlation, pointed out that "unless the 



' Reprinted (by permission), with some emendations by the author, from 

 the CoUieni Guardian, vol. cxiv, No. 2966, November 2, 1917, pp. 839-40. 

 2 Proc. Roy. Phys. Soc. Edin., vol. xii, p. 222, 1894. 



