chap, xxxv CONNECTION OF DYKES WITH SILLS 
section in connection with the history of intrusive sheets, and also to cite 
from the neighbouring island of Eaasay another good example of the same 
relation between dyke and sill. 
Sedgwick, in the paper above quoted, gave an account and figure of 
the expansion of the Cleveland dyke at Bolam, to which allusion has 
already been made. He showed that from a part of the dyke which is 
unusually contracted a great lateral extension of the igneous rock takes place 
on either side over beds of shale and coal. While in the dyke the prisms 
are as usual directed horizontally inward from the two walls, those in the 
connected sheet are vertical, and descend upon the surface of highly 
indurated strata on which the sheet rests. 
The most important examples known to me are those which occur in 
the coal-field of Stirlingshire. In that part of the country, the remarkable 
group of dykes already referred to, lying nearly parallel to each other and 
from half a mile to about three miles apart, runs in a general east and 
west direction. From one of these dykes no fewer than four sills strike off 
into the surrounding Coal-measures. The largest of them stretches south- 
wards for three miles, but the same rock is probably continued in a 
succession of detached areas which spread westwards through the coal-field 
and circle round to near the two western sheets that proceeded from the 
same dyke. Another thick mass of similar rock extends on the north side 
of the dyke for two and a half miles down the valley of the river Avon. 
These various processes, attached to or diverging from the dyke, are 
unquestionably intrusive sheets, which occupy different horizons in the 
Carboniferous series. The one on the north side has inserted itself a little 
above the top of the Carboniferous Limestone series. Those on the south 
side lie on different levels' in the Coal-measures, or, rather, they pass trans- 
gressively from one platform to another in that group of strata. 
No essential difference can be detected by the naked eye between the 
Fig. 250. — Section to show tlie connection of a Dyke with an Intrusive Sheet, 
Stirlingshire Coal-field. 
a, Dyke in line of fault ; b, Sill traversing and altering the coal ; i, Slaty-band Ironstone. 
material of the dyke and that of the sheets. If a series of specimens from 
the different exposures were mixed up, it would be impossible to separate 
those of the dyke from those of the sheets. A microscopical examination 
of the specimens likewise shows that they are perfectly identical in com- 
position and structure, being chiefly referable to rocks of the dolerite, but 
distance, almost immediately breaking across the bedding so as to leave the basalt, and rapidly 
tapering until it dies out. 
