176 W.A. Richardson—A Basic Dyke at Charnwood. 
consist of alternating sheets of country rock and dyke. These sheets 
are each about 9 inches thick, and vary in number from two to four. 
The dyke material of these sheets is chilled, and this chilled phase 
also extends not less than 2 feet from the main dyke margin. 
It was not found practicable to trace the dyke across the floor 
of the quarry. In 1914, however, an old tramway incline, since 
quarried away, exposed a portion of the dyke, and was situated 
between the two exposures on the quarry face. By means of this 
exposure the reconstruction of the course of the dyke across the 
floor given in Fig. 2 was made. This sketch is not drawn to scale, 
and the marginal features are exaggerated in size to show their 
character. It will be seen that the'sheets of dyke material fill cracks 
of lenticular cross-section in the wall. These cracks die out and may 
be replaced by others along the course of the dyke. 
To the north the dyke cannot be traced, but it does not enter the 
Trias. Although nothing can be seen of it at present, it has been 
followed for a short distance southwards. 
IV. THe AGE or THE DYKE. 
The dyke, as just mentioned, does not enter the Keuper marl, and 
is therefore pre-Triassic. 
Although in composition and probable inmereloattoe character it 
strongly resembles certain Carboniferous dolerites. (a few of which 
are to be found in Midland Coal-measures), it is much more altered 
than the Carboniferous intrusions. Moreover, it has shared the 
cleavage of the surrounding rocks, and must be considered as 
belonging to the pre-Cambrian suite. It should be noted, however, 
that the well-developed chilled marginal phase points to the country 
rock being quite cold at the time of the dyke intrusion. 
V. Tue ORIGIN oF THE MARGINAL LAMINATION. 
The dyke occurs to the west of the main Charnian anticlinal axis, 
and has the hade and direction of the chief faults of the district. 
There is no evidence of movement, and the included sheets of 
porphyroid show no signs of shearing or brecciation. 
There are three possible explanations of the phenomenon. 
1. Contraction of the dyke on consolidation may have cracked 
the sides and provided means of further intrusion. It would seem 
that several successive contractions were necessary each of the same 
order of magnitude, and this does not seem reasonable. Moreover, 
as in the case of certain ““ multiple ”’ dykes, it might be expected 
that the junction of the dyke and the country rock would provide 
the readiest line of fracture. 
2. The other cracks may have been formed by a renewal of tension 
at a later time. Here, again, one would expect the line of junction 
to be the most likely plane of weakness. Examination in the field 
and of the slices at the margin suggests, moreover, that the intrusion 
and consolidation of all the dyke material took place concurrently. 
