217 
1903-4.] Mr J. G. Goodchild on Intrusive Rocks. 
short distance. It may well have been the case that a difference 
of relative temperature of the country rocks and the magma at the 
part where the dyke passes into the form of a sill may have had 
something to do with the change of direction. (See fig. 22.) 
The fact just referred to suggests the question, why should 
the same magma eat its way in a horizontal plane at one part and 
■at another within the same type of country rock make its way 
aipwards in a nearly vertical plane 1 Possibly the answer to the 
•question may be that the magma was injected from below obliquely 
upward and outward from the focus, and that its course, as a 
whole, has really followed the oblique direction ; but as it tra- 
versed strata of very varying degrees of resistance to the thrust, 
the magma eats its way upwards in a zigzag manner, forming a 
sill on one platform, then going off as a dyke, again as a sill, and 
so on (see fig. 27, p. 226). The phenomenon may be illustrated by 
attempting to scarp a fluted surface by drawing the end of a walk- 
ing-stick in an oblique direction across the flutings. The stick will 
run along one of the flutings, make a jump to the next, along that 
again in a line nearly parallel to the first one, and so on. This is 
what is above referred to as “ trailing,” which is a phenomenon of 
■common occurrence wherever a newer set of faults crosses an older 
set in an oblique direction. 
On the view just set forth, the abundant Tertiary dykes of 
North Britain may be represented by sills at no great depth below 
the surface, and need not be supposed to extend downwards to any- 
thing like the depth with which they are credited. 
A few additional examples, out of a great many that might be 
selected from amongst Scottish writers on Geology, will now be 
referred to, in which those writers have figured the relationship 
which actually exists between an intrusive rock and the rocks it 
invades. For this purpose I give a list selected from Sir 
Archibald Geikie’s Ancient Volcanoes , and his two recently-issued 
memoirs on the Geology of Fife ; the references preceded by an 
asterisk are particularly noteworthy : 
Ancient Volcanoes of Great Britain , vol. ii., Figs. 238, 241— 
245, 248, 249, 251, *255, 304, *322, 323, 329, 349, 351, 353-5, 
■361, 371, 380, *381. “Geology of Central and Western Fife ” 
