268 THE CRETACEOUS ROCKS OF BRITAIN. 
Gault. . . . The facies from this bed is remarkably like that of the 
Greensand seam in the Gault (zone xii.) at Folkestone.” 
Ostracoda. — Valves of Ostracods were found in the samples 
from Bookham, Maiden Bradley and Ventnor, but were not 
common in any of them, They appear to be more numerous in 
the Cambridge Greensand, and Mr. Chapman has recorded 45 
species from that deposit. * The commonest are species .of 
Cytherella , Cytheropteron and Cythereis . 
Examination of the Eesidues. 
The Detrital Minerals. — These are coarsest at Maiden Bradley, 
where the largest grains measured 3 mm. and more in their 
longest diameter, the average size being * 47 mm. in the higher 
example of the Chloritic Marl in this locality. 
They diminish in size as well as in number both to the south¬ 
east and to the north-east, their average longest diameter in the Isle 
of Wight being T2 mm., Folkestone TO mm., Shouldham ■055mm. 
Mr. Teall has kindly furnished the following notes on the detrital 
minerals which occurred in these residues after treatment of the 
specimens of the Basement beds with acid solution :— 
In addition to glauconite, more or less oxidised pyrite or mar- 
casite, and the remains of arenaceous Foraminifera, these residues 
contain variable amounts of foreign clastic material. 
Apart from variations in the size of the grains, and the total 
amount, this material does not show any marked variation whe n 
the samples from different localities are compared. It may there¬ 
fore be described in general terms. 
Quartz is by far the most abundant mineral. The grains are 
often colourless, but they are not unfrequently tinged with brownish 
or greenish tints. They contain the inclusions so common in the 
quartz of granitic and gneissose rocks. Felspar and white mica 
are also present, but only in very small quantities. Microcline 
has been observed in one or two cases, and oligoclose is probably 
also present, although it has not been definitely determined. When 
the residues are placed in a diffusion column of cadmium ferrotung- 
state the felspars float either with or above the quartz. Basic 
felspars are absent. 
All the fine-grained residues contain a considerable assortment 
of heavy minerals. Garnet, staurolite, zircon, rutile, ilmenite, and 
magnetite were observed in the deposits from Cerne Abbas, Ventnor, 
and Shouldham. Tourmaline was also recognised in the deposits 
from each of the above-mentioned localities, except that from Vent¬ 
nor, and if a larger amount of material had been available it would 
probably have been found there also. 
It follows from the facts mentioned above that the foreign clastic 
material has been derived from the disintegration of plutonic and 
metamorphic rocks. No minerals characteristic of volcanic recks 
have been observed. (J.J.H.T.) 
* Ann. Mag. Nat. Hist., Ser. 7,‘ Vol. ii. p. 3'31 (1898). 
