134 Mr Rastall, The Mineral Composition of 
a mineral grain covered by an opaque skin of haematite or limonite~ 
cannot be identified, and is therefore useless, unless this skin is 
removed. It is necessary therefore to run the risk of coe 
some of the less stable minerals. 
The separation of the heavier constituents was effected by the 
heavy-liquid method, the particular liquid used being bromoform, 
which when pure has a density of about 2°93. This removes all 
the quartz and felspar and nearly all the glauconite. The density 
of this latter mineral, however, appears to be variable since some 
green grains occasionally sank in bromoform. According to the 
best authorities the density of glauconite is about 2°3 only. It is 
evident therefore that this point requires further investigation. 
It is unnecessary here to describe the precise form of apparatus — 
employed, which was of the most simple nature. A very little 
experience showed that any attempt at a quantitative determina- 
tion by the heavy-liquid method must be absolutely unreliable, 
owing to the impossibility of ensuring a complete separation. In 
some cases it was found necessary to treat the heavy residues again 
with acid in order to get rid of the pellicle of iron oxide which is 
such a persistent feature in these sands: in a few instances this 
was unnecessary. A part of each residue was then mounted in 
Canada Balsam in the usual way for microscopic examination, and 
the rest reserved for any other tests required. 
Detailed Description of the Sands. 
A large number of samples were examined by the above 
methods, but many of them were very much alike in their general 
characters, and it is unnecessary to describe them all. The following 
cases are therefore selected as typical examples of the sands and 
finer portions of the gravels of different ages. 
I. The Plateau Gravels. 
Pit on Golf-Links, summit of Gog-Magog Hills, 
200 ft. above O.D. 
The beds exposed in this pit consist partly of gravel and partly 
of a very fine-grained sand in thick beds, which often show marked — 
contortion. The larger constituents of the gravel comprise a great 
variety of far-travelled rocks, and from pits closely adjoining the 
present one, which were open a few years ago, a large number of 
interesting rock-types were recorded, including abundant igneous 
rocks, both Scotch and Scandinavian, Carboniferous Limestone and 
Millstone Grit, several varieties of Jurassic sediments, Carstone 
and Hunstanton Red Rock, together with shells of Gryphea bored 
