228 F. Eutky — Ironstone near Dover. 



collected years ago at Capel, near Dover, shows, under the micro- 

 scope, that it is composed almost exclusively of quartz-sand and 

 limonite. The latter substance, which appears of a deep brownish- 

 red colour in reflected light, does not exhibit any pisolitic or other 

 structure, but is simply an amorphous cement which holds together 

 numbers of minute grains of quartz-sand, some of which are rounded, 

 while a large proportion of them are sharply angular. Here and 

 there a small grain of felspar, of somewhat turbid aspect, shows in 

 convergent light a dark brush or the cross of a bisectrix, which 

 separates into dark brushes on rotation, but such grains are of 

 comparatively rare occurrence, and the sand of the deposit may, 

 therefore, be regarded as an almost pure quartz-sand. In these 

 grains fluid lacunee, containing bubbles, may be detected under high 

 powers, and occasionally small rod-like crystals may be seen as 

 inclusions in a quartz-grain. These appear in many cases to be 

 rutile. 



The question naturally arises whence this fine quartz-sand came. 

 We meet with none of the grains which are composite in character, 

 that is to say, there are no fragments composed of agglutinated 

 grains such as one would expect to meet with if the material had 

 resulted from the immediate disintegration of a quartzite or schist, 

 neither are these grains composed of two or three adherent crystals 

 of diifei'ent minerals such as would come from the immediate dis- 

 integration of a granite or other eruptive rock. The fact that there 

 is comparatively little attrition involved in the transport by water 

 of such minute grains would sufficiently account for the sharply 

 angular character of a large proportion of them. There is, con- 

 sequently, nothing to prove that they may not have travelled a long 

 distance. That they were originally derived from the disintegration 

 of eruptive rocks is tolerably certain, but it is more than probable 

 that such material subsequently entered into the composition of some 

 sedimentary rock, a sandstone, or a quartzite, and that it is from the 

 disintegration of some such rock that the sand in question has been 

 more immediately derived. 



According to Dr. Sorby, about one-third of the grains constituting 

 the Hastings Sand, at Hastings, are rounded by attrition.^ This 

 seems to be about the proportion of rounded grains in the sand of 

 the ironstone we are now considering, and it does not appear by any 

 means improbable that these sand-grains may have been derived 

 either from the detritus of certain beds in the Greensand of the 

 South-eastern Counties, in which Dr. Sorby detected an average of 

 from ^ to 1^ of rounded grains, or from the Hastings Sand. The 

 denudation of the Weald would have provided vast quantities of 

 such material, and Prof. Prestwich considers " that the anticlinal 

 arch over the Wealden area had been formed and the Chalk greatly 

 eroded, so that in some places the Greensands below the Chalk had 

 been laid bare, before even the lowest of the Eocene beds was 

 deposited." ^ It, therefore, appears highly probable that the sands 



1 Anniversary Address, Geol. Soc. Lond. 1880, p. 34. 



2 Jukes' "School Manual of Geology," 4th edition, edited by A. J. Jukes- 

 Browne, p. 334. 



