1903-4.] Mr J. G. Goodchild on Intrusive Rocks. 
197 
Some Field Evidence Relating to the Modes of Occur- 
rence of Intrusive Rocks, with some Remarks upon 
the Origin of Eruptive Rocks in General. By J. G. 
Goodchild, of the Geological Survey, F.G.S., F.Z.S., 
Curator of the Collection of Scottish Mineralogy in the 
Edinburgh Museum of Science and Art. Communicated by 
R. H. Traquair, LL.D., M.D., F.R.S. 
(Read Dec. 5, 1903 ; MS. received Jan. 6, 1904.) 
SYNOPSIS. 
1. Introduction, pp. 197-199. History of previous opinion, pp. 199-202. 
Evidence bearing upon the question whether intrusive rocks displace or 
replace the rocks they invade, pp. 202-218. Basic sills in sandstones, 
pp. 202-204 ; in shales, pp. 205-207 ; in limestones, p. 208; in coal seams, 
pp. 208-210. Basic dykes in the same connection, pp. 211-212. Acid 
intrusions, pp. 212-213. Anomalies in the mode of occurrence of dykes 
discussed, pp. 213-217. Relation between dykes and sills, p. 217. 
Evidence cited from other sources, pp. 217-218. Summary of the 
author’s conclusions, pp. 218-226. 
It is commonly believed by geologists, as well as by coal miners, 
that the inner faces of the rocks which enclose intrusive masses 
were at one time in contact, and that each of these surfaces is the 
counterpart in form to the other, from which it has been severed 
by the forces to which the injection of the intrusive mass was due. 
In the case of a sill, for example, this belief implies that the rock 
floor below the sill and the roof above it were in unbroken 
contact at some time before the sill was intruded, and that the 
floor and the roof have been forced apart to a distance equal to 
the thickness of the intrusive mass. In like manner, so it is 
believed, the walls right and left of a dyke are supposed to have 
been thrust apart from their original position. In other words, it 
is evidently the common belief that these intrusive rocks, 
whatever their volume may be, have added that volume to the 
rocks they invade. To put this statement into yet another form, 
it is evidently believed that two seams of coal, or beds of black- 
band, or of oil shale, which occur under normal conditions at ten 
