274 
THE CRETACEOUS ROCKS OF BRITAIN. 
B. The Quartziferous Limestones of Devonshire. 
These beds are of abnormal character, and some of their macro¬ 
scopic features have been described on p. 130. The lowest part 
of the deposit is generally very coarse-grained, many of the quartz- 
grains being as large as a pea. Above this is a hard •'calcareous 
sandstone (Bed 11 of Mr. Meyer), consisting chiefly of shell- 
fragments and quartz-sand compacted by calcite into a hard rock. 
Still higher is a hard white limestone through which large quartz- 
grains are plentifully scattered ; this is No. 12 of Meyer and B. 
of our sections. No. 13 (our bed C) is a sandy glauconite chalk, 
[Bor microscopic structure, see p. 299.] 
l.—Hard Limestone overlying Greensand west of Beer Harbour. 
69 ’ 95 grammes of this material, after treatment with 20 per 
cent. HCL, yielded ; 4 - 97 grs. = 7’116 per cent. ; 
Fine residue, 1'25 grs. = l*78 per cent.; 
Heavy residue, 3*73 grs. = 5*33 per cent.; 
Thus: CaC0 3 = 92*88 per cent. 
1. Of the heavy residue that above 1 mm. in diameter weighs 
1 * 09 grs. A noticeable feature in it is the extent of the silicification 
and the abundance of the detrital materials. The silicified frag¬ 
ments consist of— 
(i a ) Brachiopod fragments. One Kingena had been completely 
silicified, even parts of the loops being replaced, but in most 
cases the shells were in fragments, and were recognised by 
the pores distributed over the surface, comparison being 
made with unchanged Brachiopoda. Viewed from the 
side, some are seen to be delicately costate, and they form 
hollow rectangular masses, only one wall of which is porous. 
They are sometimes of a faint red tint, having been coloured 
by ferruginous oxides. 
(b) Very perfectly preserved hexagonal meshes, due to the inter- 
prismatic silicification of Inoceramus- shells, are abundant, 
showing columnar structure when viewed in side section. 
The detrital minerals consist almost entirely of quartz, in rounded, 
colourless, translucent, whitish, green, and red-tinged pebbles 
and a number of others of most irregular form. The largest is 
about 1'5 mm. diameter. Masses of limonite are present, and glau¬ 
conite in aggregations, which show but little trace of organic 
structure. 
2. Residue of * 5 to 1 mm. diameter. One of the special features 
is the abundance and extreme angularity of the quartz, most 
of the grains of this mineral showing distinct traces of the original 
crystal form. In some the prism edges are only just beginning to 
be rounded off, and the pyramid faces are still evident.* Even 
where the pebbles are definitely rounded, they are oblong in shape, 
and have probably not long been derived from idiomorphic forms. 
Flat flakes are not uncommon, and, in addition to the translucent 
ones, there are a number of others of a yellowish colour. 
* Some of these may be secondary growths of quartz. 
