416 
DR. G. J. HINDE OX BEDS OF SPONGE-REMAIXS IX THE 
it is entirely negative between crossed nicols. The empty spicular cavities, to which 
the porous nature of the rock is due, appear in fractured surfaces as very minute, 
short, straight tubes or open canals from ‘15 to ’4 mm. in length by ’03 mm. in width. 
They are undoubtedly the casts of acerate and trifid siliceous spicules, and they are 
sufficiently numerous to show that the silica derived from the solution of the contained 
spicules can have produced the colloid material of which the rock principally consists. 
There can be no doubt that this malm rock is derived from the spicules of siliceous 
sponges. 
Messrs. Way and Paine first called attention to the fact that the Farnham malm 
and firestone contain a large amount, varying from 25 to 75 per cent., of soluble 
silica, but they do not appear to have noticed the peculiar globular form in which the 
silica has been deposited, and* they deny its organic origin. 
Selborne, Hampshire. — The malm rock is well shown in the quarries at this village, 
where a section of 15 feet (4\5 m.) is exposed ; also in road-cuttings and on the 
surfaces of the adjoining fields. It is of a much harder and more compact character 
than that at Farnham, and contains a fair proportion of calcite, which renders it more 
suitable as building material. Spicules and spicular casts abound in it. Some of the 
casts of spicules in the more calcareous beds have been refilled with crystalline calcite, 
whilst in sections of the more siliceous beds the spicules are seen thickly grouped 
together, surrounded by globular silica and also by transparent chalcedonic silica. 
The spicular canals are infilled by glauconite, which remains intact after the spicular 
walls have been dissolved. 
Gilbert White, in the ‘Natural History t of Selborne,’ referring to this rock, 
states that it is but little removed from chalk in appearance, but seems so far from 
being calcareous that it endures extreme heat. It was known as the white malm, in 
contradistinction to the chloritic marl, which was called the black malm. Its organic 
origin does not seem to have been suspected. 
Wallingford, Berkshire. —The malm rock is not limited to the northern margin of 
the Weald, for at Wallingford, which is situated about thirty-two miles to the north¬ 
west, there are exposed beds of siliceo-calcareous rock of the same character as those 
already described round the Wealden area. The rock is hard, and calcite is only 
occasionally present in it, but there is a greater relative amount of quartz-sand and 
other mechanically derived constituents than in the malm of the Wealden district. 
The sponge-remains are either as empty casts or of amorphous silica, and the siliceous 
matrix is partly in the globular form. 
* “ On the Silica Strata of the Lower Chalk.” ‘ Journal of the Royal Agricultural Society,’ vol. sir., 
p. 243. These authors state that it is “not infusorial, for with the exception of a few foraminifera uo 
traces of animal life can be observed in the rock by microscopical examination.” They attribute its 
formation to “ aqueous decomposition resulting from the contact of silicate of lime in solution, derived 
from the older rocks, with carbonic acid, produced either by vegetable and animal decay or by 
volcanic agency, so that at one and the same time carbonate of lime aud gelatinous or soluble silica 
would be formed.” 
f Bell's edition, p. 3. 
