454 HE. Neaverson—Foraminifera of the Hartwell Clay. 
of low and medium refractive index, which is very characteristic of 
silicification. Carbonates are almost wholly absent, and neither 
apatite nor pyrite have been detected. ‘The pyroxenes and also the 
opaque minerals are largely replaced by brown limonitic material, 
but cores of the fresh minerals still remain, together with chloritic 
borders around the pyroxenes and cream-grey streaks of leucoxene 
traversing the opaque minerals. The interstitial matter, as before, 
remains unaltered. 
Specimen No. 8 (surface of the dyke, near margin—Exposure II) 
exhibits the effect of weathering on material of the same type as 
No. 7. Calcite and dolomite have both been leached out almost 
completely, while limonite has been leached out in certain parts 
and concentrated in well-marked bands parallel to the exposed 
surface. 
It is clear that the two processes of alteration are of the same 
character as those involved in the infilling of the amygdales found in 
the Whin Sill and the Hett dyke. They represent a late-magmatic 
(paulopost) and probably hydrothemal stage of the waning magmatic 
activities, and have nothing to do with weathering, as commonly 
supposed, although at the same time the material in question is 
deeply weathered. 
The alteration to the margins is evidently a consequence of the 
contraction of the dyke. The residual liquors, wherever situated, 
would tend to become concentrated and to move upwards along the 
channels offering least resistance ; that is to say, along the contrac- 
tion fissures which developed along the margins. 
EXPLANATION OF PLATE VIII. 
Magnification = 22°5 diameters. 
Fia. 
1.—Typical field of the dolerite of the Wackerfield dyke. Specimen No. 6 
from Exposure II. See p. 446 for description. 
2.—Typical field of the dolerite of the Little Whin Sill, Stanhope. The minerals 
are identicai with those of the Wackerfield dyke, except that the felspars 
are slightly turbid. In texture the only difference is that here the felspar 
laths are less uniformly dispersed. 
3.—Corroded phenocryst of bytownite in the Wackerfield dyke. Specimen 
No. 1 from Exposure I. See p. 447 for description. 
The Foraminifera of the Hartwell Clay and 
Subjacent Beds. 
By EH. Neaverson, M.Sc., F.G.S. 
(PLATE IX.) 
INTRODUCTION. 
[LP URWG a temporary cessation of work on the Ammonites of 
the Hartwell Clay, the writer was asked to identify a series 
of Foraminifera found by Mr. EH. Hollis of the Bucks. County 
Museum, Aylesbury, in washing a sample of clay from Hartwell. 
Other samples were collected and examined from all the available 
