104 Miss MW. K. Heslop—Pre-Tertiary Dyke, Usway Burn. 
Trigonia-Grit, but then, as the surface of the ground declined in the 
field just to the north of the Sanatorium-bungalows, the oyster-layer 
on top of the Upper Zregonia-Grit, the thin representative of the 
Notgrove Freestone (with bored and oyster-covered top-layer), and 
Gryphite-Grit were successively proved. Then came a small fault, 
the Fullers’ Earth being let down against the ‘Grit’. The Fullers’ 
Earth, is, however, only a small mass, for the Clypeus-Grit rises up 
from beneath it but a few yards farther on, and is excellently exposed 
in the vertically-cut bank at the back of the bungalows. 
I am indebted to Mr. J. W. Gray, F.G.S., of Cheltenham, for much 
of the information upon which this paper is based. He obtained the 
details of the Upper-Lias deposits exposed in the well, and collected 
the fossils that afforded the information as to their date. 
IIJ.—Nores on a pre-Tertrary Dyxr on tur Usway Born. 
By Miss M. K. Hustor, M.Sc. 
(PLATE XL) 
DYKE of pre-Tertiary age is exposed in the lower course of the 
Usway Burn, a tributary of the Coquet. It is intruded among 
the Cheviot igneous rocks, and crosses the burn in a somewhat north- 
easterly direction. In hand-specimens the rock appears black or 
dark brown with a sub-vitreous lustre, and contains porphyritic 
crystals which are quite visible to the unaided eye. It weathers 
a bright red colour and even the fresh portions are streaked with red. 
veins of agate. 
In his paper on the Cheviot Andesites and Porphyrites,’ Dr. Teall 
mentions a. rock allied to pitchstone-porphyrite, which is exposed on 
the Coquet about a quarter of a mile above Windy Haugh. The 
description. he gives of it would serve very well for the Usway Burn 
Dyke, but he deals chiefly with the nature of the prevailing pyroxene, 
and not at all with the minute structure of the rock. 
Under the microscope the porphyritic nature of the Usway Burn 
Dyke is at once evident, large phenocrysts of felspar and pyroxene 
being embedded in a rather dark glass crowded with elementary 
crystals. Between these two extremes is a medium-sized set of 
crystals, which may be referred to as the ground-mass generation. 
There are some rounded masses of a black oxide of iron which bear 
such evident traces of corrosion that they must be classed, with the 
porphyritic elements, among the products of an earlier period of 
crystallization. It is extremely difficult to determine the order in 
which the crystals were formed—particularly as the limits of the 
generations are very ill-defined—but there can be little doubt that the 
apatite needles and small grains of iron oxide, which are both included 
in the porphyritic crystals, are the oldest secretions of the magma. 
The porphyritic pyroxenes are older than the felspars by which they 
are frequently enclosed. So that in this, the first generation, the 
1 «Notes on the Cheviot Andesites and Porphyrites,’”’ by J. J. Harris Teall, 
M.A., F.G.8.: Gor. Mac., April, 1883. 
