1903 - 4 .] Mr J. G. Goodchild on Intrusive Bocks. 
199 
appears to lead. In the latter part of the paper, while passing 
additional facts in review, I shall venture to submit for the 
consideration of field geologists * an hypothesis which appears to 
me to he in full harmony with the facts. 
The question Avhether intrusive rocks displace or replace the 
rocks they invade has often been raised before. A brief notice of 
two or three of the more important papers dealing with the 
subject cannot he out of place, and accordingly they are given 
here. 
In 1852 or 1853 the late Prof. J. Beete Jukes wrote in the 
Geological Survey Memoir , “ On the Geology of the South 
Staffordshire Coal-field,” pp. 246-7, as follows : “ I was assured 
also by almost every one engaged in the works of this neighbour- 
hood that, notwithstanding the variation in thickness of ‘The 
Green Bock’ [a basic sill], there was no change in the total 
thickness of the measures ; that, for instance, the thickness 
between the New Mine Coal and the Blue Flats Ironstone 
remained the same, whatever might be the variation in the 
thickness of ‘The Green Bock.’ In other words, it was affirmed 
almost universally that ‘ The Green Bock ’ not only intruded 
between the measures, but obliterated [the italics are the author’s] 
a mass of beds equal to its own thickness.” Jukes then goes on 
to express a doubt about the miner’s conclusions • nevertheless, on 
the next page (247) he adds: “At Union Colliery, north of 
[Walsall], the Bottom Coal is cut out entirely by ‘green rock.’” 
I do not give the evidence cited by Jukes in support of his own 
view, as the fact that he was informed of evidence of the trap 
cutting out the coal is all that need be referred to here now. 
There may have been other evidence published before that, or 
since, of which I have at present no information. But, in 1867, 
Mr Hughes (now the Woodwardian Professor of Geology at 
Cambridge, wrote as follows in a review of Nicholson’s “ Essay 
on the Geology of Cumberland and Westmorland,” Geol. Mag., 
dec. i., vol. v., pp. 466-7 (1868) : — 
“ One point seems often to come out from a careful examination 
of a granite mass. The granite seems to replace a certain portion 
* The questions raised are of a 'pdro graphical as distinguished from litho- 
logical character. 
