H. A. Baker — Quartzite Pebbles of the Oldhaven Beds. 65 



from the operation of what has been termed " atmospheric meta- 

 morphism ". The rocks are really sandstones or grits which have 

 been cemented as the result of a metasomatic change — the addition of 

 fresh silica as a cementing medium. The name " quartzite ", as 

 applied to the rocks, is legitimate by virtue of the definition that 

 a quartzite is a rock composed chiefly of quartz grains with the 

 addition of a quartz cement. Yet, in view of the irregular and 

 inconstant character of the cementation the rock may, with equal 

 propriety, be frequently described as sandstone (where the grains 

 are well rounded) or grit (where the grains have preserved their 

 angularity). It is frequently possible, in diiierent parts of the same 

 slice, to apply the names quartzite, sandstone, and grit with equal 

 justification. 



In all the slices examined by the writer no evidence of secondary 

 outgrowth from the quartz-grains was to be observed, and there 

 was no sign of optical continuity between the original grains and 

 the secondary silica. The latter was often amorphous, apparently 

 colloidal, and generally dirty with disseminated iron-oxides. 

 Approach to a quartz-mosaic structure was seen only rarely and 

 locally, and was but imperfectly developed. Where observed, 

 the interlocking grains were of small size, and the feature was 

 apparently developed only in the secondary silica. In the examina- 

 tion of the slices the j)resence of other minerals such as tourmaline, 

 kyanite, staurolite, zircon, rutile, glauconite, felspar, and iron- 

 oxides was noted. With a view to the further examination of these 

 minerals some of the pebbles were crushed, and minerals possessing 

 a specific gravity of more than 2" 80 were isolated by the use of 

 Thoulet's (Sonstadt's) solution. It was then seen that the heavy 

 residue was composed essentially of the assemblage of minerals so 

 characteristic of the heavy residues of the Woolwich and Reading 

 Beds. The writer has recently examined a considerable number 

 of heavy residues from these beds, from various localities on the 

 southern side of the London Basin, and he has found the mineral 

 assemblage to be one of remarkable constancy of character. The 

 number of heavy minerals occurring is not large, but the whole 

 suite is invariably present and the minerals always exhibit the 

 same individual characteristics and occur in the same relative 

 •order of abundance. The assemblage of minerals (of specific 

 gravity greater than 2-80) is chiefly characterized by the presence 

 of kyanite, tourmaline, zircon, stauroUte, rutile, ilmenite with 

 leucoxene, magnetite, limonite, and glauconite. Others, such as 

 colourless garnet, haematite, pyrite, anatase, and very rarely 

 andalusite, occur. With regard to the glauconite, which is always 

 present, no definite idea of its degree of abundance can be arrived 

 at, since the amount of iron present in the mineral appears to vary, 

 and the consequent variability of its specific gravity governs its 

 behaviour in the heavy liquid. 



The heavy residues from the quartzite pebbles were precisely 



VOL. LVII. — NO. II. 5 



