4.4.6 Dr. A. Holmes and Dr. S. Smith— 
dyke. The laccolith has considerable thickness but is not extensive ; 
it has tosome extent the characters of a cedar-tree laccolith. 
Attenuation of the dyke takes place in proximity to horizontal 
injection in all cases. : 
These statements show that the Wackerfield Dyke outcrops on 
the south side of the more important intrusion but coincides in its 
extent with a gap in the surface continuity of the great dyke and 
that both dykes give rise to horizontal offshoots in the same region 
and in the same manner. 
The two dykes are petrologically quite dissimilar and can have no 
direct genetic relationship. Without anticipating unnecessarily the 
subsequent section of this paper it may be stated that the Cleveland 
Dyke has an andesitic character and is of Tertiary Age, whilst the 
Wackerfield Dyke is a dolerite mineralogically identical with the 
Whin Sill but of a finer texture. It also resembles the Hett Dyke 
both in texture and composition, but perhaps less closely so than 
it does the Whin Sill. Consanguinity between these dykes and the 
Whin Sill may fairly be assumed and a Carboniferous age argued. 
The Hett Dyke, which has the same E.N.E.—W.S8.W. bearing as 
the Wackerfield Dyke, has been traced from the vicinity of Durham 
to that of Middleton-in-Teesdale, some 25 miles, and has been 
quarried at many points and for considerable lengths along its 
course. Itis a narrow dyke—at Howden le Wear, where it was 
followed by one of us for some 2 miles, it only varied in its width 
from 12 to 18 feet—but runs in a remarkably straight line across 
the country it traverses. 
PETROLOGY. 
Megascopic—The fresh rock of the Wackerfield Dyke is 
remarkably uniform in appearance throughout the different 
exposures. It is a dark-grey finely crystalline rock which closely 
resembles the most commonly occurring variety of the Whin Sill, 
but the texture becomes somewhat finer near the margins of the 
intrusion. The rock is altered in places to a buff-coloured material, 
and between this and the typically fresh rock are intervening stages 
in which the colours are dull green, purple, and tints of grey. This 
alteration, although affecting more particularly their contacts, 
penetrates deeply and irregularly into the dyke and sills. The nature 
of the alteration is discussed later, and for the present only the fresh 
rock will be considered. Some dozen specimens were collected for 
study, and all show an even-grained texture practically free from 
phenocrysts. Careful examination with a lens reveals a few sparsely 
distributed phenocrysts of felspar, the longest dimension being 
about three millimetres, against an average for the bulk of the rock 
of 0-8 millimetre. The specific gravity varies from 2°90 to 2°98, 
the actual values for particular specimens being as follows :— 
No. 1 6 8 9 LOS oe ISL 
2°96 2°96 2°93 Pork 2°90 2°98 
