THE GEOLOGY OF THE ENGLISH CHANNEL. 153 



at the centre, when dried weighed 51 grains ; it was boiled in water for 

 twenty minutes, allowed to remain in the same water until cold, when 

 it was taken out, wiped, and found to weigh 62^ grains. The specific 

 gravity of the remaining mineral was ascertained to be about 2"56. If 

 we assume that all the lost material was chalcedonic silica we have to 

 multiply the weight of water absorbed, 11 1 grains, by the specific 

 gravity of chalcedony, say, 2 '3, in order to ascertain the loss of the 

 original rock by solution ; this gives us about 26 1 grains or somewhat 

 over 33 % of the mass of the original flint. From the fact that 

 the specific gravity of the residue is less than that of quartz, although 

 some calcite also occurs in the rock, it may be assumed that not all 

 the silica yet remaining is crystalline. 



These porous altered flints effervesce, some more freely than others, 

 on treatment with acid, but maintain their outward form. The same 

 specimen on which the above determinations were made lost 3| grains 

 in weight after prolonged stay in dilute acid. (This loss includes a 

 very small amount of silica and the merest trace of iron.) It would 

 thus appear that the unaltered rock had contained at least 4 % of 

 soluble carbonate. In some instances this is certainly exceeded. 



M. 62b. S. 25" W. Edd., 46-4 miles. 



This specimen was selected for especial microscopic examination ; it 

 is a somewhat chert-like black flint, the outer portion altered as above 

 described. The first section was made from the black part of the 

 pebble, and shows the flint to be nothing more than a silicified 

 chalk. Foraminifera and shell fragments, all still carbonate of lime, 

 crowd the slide, and there are occasional quartz grains and some of 

 glauconite. The matrix is partly crypto-crystalline, and so intimately 

 is the crystalline mixed with the isotropic that practically all the 

 silica ground-mass gives some reaction with polarised light. A second 

 slide, cut from the decomposed part of the rock, shows a crypto- 

 crystalline ground-mass of silica crowded with forms in calcite after 

 organic matter. Foraminifera quite numerous, the chamber walls well 

 defined, but the original structure mainly, if not entirely, replaced 

 by granular calcite. An occasional fragment of some larger shell 

 (molluscan) appears to have retained pretty well its original structure. 

 Glauconite grains occur freely, and some of the foraminifera are 

 infilled with this material. 



Finally, to remove any possible ambiguity as to the presence of 

 carbonate of lime, a third section was prepared, also from the decom- 

 posed portion. After this had been ground down to the requisite 

 transparency one-half of it was varnished with Canada balsam, and 

 the whole section dipped in dilute acid ; a brisk but brief effervescence 



