18 
PROCEEDINGS OF SOCIETIES. 
period, while the rocks were yet undisturbed,—and that other varieties 
of those rocks were poured out as molten sheets at the surface either in 
air, or under water, accompanied by those mechanically transported ejec- 
tamenta, or debris, which we call “ ash;” there being every gradation 
from those igneous materials into purely aqueous slates and gritstones. 
The intrusion of the first-named must unquestionably have been ac¬ 
companied by a certain amount of metamorphic effect on those rocks 
with which they came in contact; neither are the contemporaneous traps 
and ashes always devoid of metamorphic characters, since subsequently 
intrusive rocks would exercise a more marked effect on these than on 
purely argillaceous or arenaceous rocks. They may have in many in¬ 
stances partaken both of a local metamorphosis, derived from the con¬ 
tact of intrusive rocks, and also of a more general metamorphic effect, 
which may have either been the result of a long continuance of the 
elevated temperature which they would attain to when buried under 
several thousand feet of other rock, or, perhaps may have been the effect 
of mechanical pressure, or of chemical actions and reactions, which may 
or may not have-been accompanied by heat. 
Viewed from this point, the problem becomes a very complex one, 
since we have a set of rocks, to begin with, of a very various and com¬ 
plicated character, affording almost every gradation from a molten rock 
to a mud, of the deposition and formation of which we have to un¬ 
ravel the history, to determine, first, which were contemporaneous, and 
which intrusive; secondly, of the contemporaneous, to discover which 
flowed as molten sheets, and which were deposited as “ashes” blown 
into the air, or as fragments worn by the water from previously conso¬ 
lidated masses; and thirdly, of the intrusive we have also to inquire 
which were erupted previously to, and which subsequently, to the for¬ 
mation of the contemporaneous ones. * 
We have then to consider the various conditions in which those 
igneous rocks, together with their associated aqueous deposits, have been 
placed since their formation,—to take into account that they have been 
buried many thousand feet deep in the earth, and subsequently re¬ 
elevated and exposed at the surface in consequence of the removal by 
erosion and degradation of those thousands of feet of other rock which 
had covered them. We have to inquire whether any of the intrusive 
igneous rocks were injected during any of those subsequent periods of 
elevation and depression, and, if so, which they were, and how they dif¬ 
fer from the intrusive rocks that originally existed in the locality. 
Difficult, however, as the problem thus stated may appear, it seems 
to me that its very difficulty gives it an interest and a charm which was 
wanting in the previous views taken both of these and of igneous rocks 
in general. There is a varied history to be learned, a complicated puz¬ 
zle to be unravelled, and our curiosity thus becomes awakened and 
aroused, and every step made in advance in the process of investigation 
becomes watched with an interest that would not be felt in a mere dry 
detail of matters of fact. 
It is for this reason that I have ventured to-night to endeavour to 
