60 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



southern front of Cam Dcarg (636 feet high). The accompanying figure (fig. 14) shows 

 what I believe to be the true geological structure of the locality, but the actual junction 

 of the dyke and sheet is concealed under the talus of the slope. I shall have occasion in 

 a later part of this paper to refer again to this section in connection with the history 

 of intrusive sheets. 



Sedgwick, in the paper already 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. 



But by far 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 



Fig. 14. — Section showing the connection of a Dyke with an Intrusive Sheet, Point of Suisnish, Skye. g, Granophyre of 

 Cam Dearg ; /, similar rock, which appears eastward under the "sill" (d) ; e, intrusive sheet of fine-grained "basalt" ; 

 d, intrusive sheet or "sill " of coarse dolerite, 200 feet thick at its maximum, and rapidly thinning out ; c, dyke or pipe of 

 finer grain than d ; b, yellowish-brown shaly sandstones, and a, dark sandy shales (Lias). 



three miles apart, runs in a general east and west direction. From one of these dykes no 

 fewer than four sheets or " sills" strike off into the surrounding Coal-Measures. The 

 largest of them stretches southwards for three miles, but the same rock is probably con- 

 tinued in a succession of detached areas which spread westwards through the coal-field 

 and circle round to near the two western sheets that proceed 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 transgressively from one 

 platform to another in that group of strata. 



No essential difference can be detected by the naked eye between the 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. 



