214 : J. J. H, TEALL——PETROLOGICAL NOTES ON 
north margin of the dyke, in the large quarry at Cockfield, and at 
the base of the intrusive sheet at Bolam. In the former locality 
the actual margin consists of a brown layer about an inch in 
thickness, and evidently rich in iron oxides. This is sharply 
divided from a light grey band in which kaolinized felspars are very 
conspicuous. This band is several inches in thickness and shades 
_off gradually into themain mass of the dyke. It does not effervesce 
with acid; but the dark and, to the eye, comparatively unaltered- 
looking rock which adjoins it does effervesce. Carbonates are thus 
seen to be present in the dark rock, but absent in the lght-grey 
band, which therefore represents a further stage of decomposition. 
My friend Mr. G. HE. Stead, of Middlesboro’, to whose kindness and 
generosity | am indebted for most of the analyses which appear in 
this paper, found the white substance occurring beneath the whin 
at Bolam to contain :— 
Per cent. 
Sra AG Ue Pe) CRE Mee 54-00 
At maria Fae eats 30°93 
krontomidesictiee« ee. 0°78 
Here Ae tc eee a BR 0-47 
NMiaerieeia Aiea ima Bs 0:74 
Potash wee) eee ee. oe. life 
Sodat G44. helices aoe 245 
Thossatals, Rif oe ee 8:95 
99°73 
It is thus seen to consist mainly of hydrated silicate of alumina, 
and to have been formed from the original rock (see analysis on 
p- 224) by the removal, doubtless as carbonates, of the iron, lime, 
and magnesia. It appears remarkable that the alkalies should have 
been left. Now the light-grey band at Cockfield is evidently similar 
in composition to this white substance, although probably the removal 
of lime, iron, and magnesia has not been quite so effectually carried 
out. The brown band which forms the actual margin is certainly 
tich in iron-oxides, so that there appears to have been here as at 
High Green (see p. 240) a removal of the iron from the interior of 
the rock and a concentration of it along a bounding surface. 
Narrow veins of calcite may occasionally be seen traversing the 
rock, and these sometimes contain pyrites. Dr. Veitch, of Middles- 
boro’, gave me a specimen of the rock showing a joint surface coated 
with a radiating fibrous zeolite, which Dr. Trechmann determined to 
be pectolite. 
Angular and subangular portions, which appear homogeneous to 
the unaided eye, are frequently seen lying in a finely crystalline 
ground-mass. As these fragments resemble very closely the rock 
formed by rapid consolidation at the sides of the dyke, I am disposed 
to regard them as having been portions of the side which were 
broken up and carried along by subsequent movements of the igneous 
matter. Inclusions of foreign rocks are not common. The only 
specimen which has come under my notice was found by Dr. Trech- 
ae 
