218 Proceedings of Royal Society of Edinburgh. [sess. 
( Meins . Geol. Survey ), Figs. 18, *20, *23-25. “Geology of Eastern*. 
Fife ” ( Mems . Geol. Survey ), Figs. 32, 60, 62. To these reference 
may be made to Mr David Burns’ diagrams relating to the Whin 
Sill which illustrates his paper in the Proceedings N. of England 
Institute of Mining and Mechanical Engineers , vol. xxvii., Plate V. 
The illustrations cited relate to a considerable variety of petro- 
graphical types, of both the intruding masses and rocks invaded. 
They include several figures of sections in which eruptive rocks 
are clearly seen to cut out coal seams — not merely by altering 
their quality, so that they have been rendered unfit for ordinary 
uses, hut by actually replacing the coal seams, in the same manner 
as many intrusive rocks occupy the place of other materials which 
have been removed, concurrently with the act of intrusion. As 
before remarked, this feature is one of considerable importance both 
from a commercial point of view and on account of its bearing 
upon the questions here under consideration. 
I commend the facts above stated to the careful considera- 
tion of all unprejudiced geologists. It must be quite evident to 
such workers, after a study of the foregoing considerations, that the 
views commonly held with regard to intrusive rocks will have to be 
modified to a very considerable extent. That must be done, what- 
ever view one may entertain with regard to how these facts have 
been brought about. It may be well to remark here that I do not 
wish the readers to understand that any other signs of mechanical 
rupture than those specially referred to do not exist ; but I 
certainly do intend to convey the idea that such evidence is of 
very much less common occurrence than most people seem to believe. 
Furthermore, I state emphatically that even in the cases where- 
there undoubtedly is evidence of a certain amount of displacement,, 
the extent of that displacement is, as a rule, by no means com- 
mensurate with the volume of the rock intruded. It appears likely 
that the degree of viscosity of the magma on the one hand, and 
the resistance presented to the intrusive force on the other, are the 
chief factors which determine the mode of occurrence of intrusive 
masses. Where a viscous, or a half-consolidated, mass is being forced 
between imperfectly consolidated materials, and under relatively 
small superincumbent pressure, it is most likely that the overlying 
