.Coarseness of Igneous Rocks — Lane. yi 
sive, if that is formed at an early stage and increases con- 
tinuously from the center. 
Extra coarse grain of intrusives like pegmatytes, may 
point to a country rock near fusion-solution, or to such 
an accumulation of mineralizers as to bring their consohda- 
tion-crystallization point down close to that of the country 
rock, which amounts to the same thing. 
. Fine grained rocks of uniform grain like aplytes seem 
to point to the following conditions : (i) small dimensions (c) ; 
which is generally the case ; (2) a considerable difference in 
condition between the consoHdation point and that of the 
country rock (i. e. u reasonably large) ; (3) but this differ- 
ence must nevertheless be less than half the initial difference 
Ca./^^) 
Suppose for instance a gabbro magma were at 1400° C. 
originally and consolidated at 1100° C. By the time the 
margin and contact zone had cooled down to 800° C. there 
would be a tendency to shrink which might be relieved by 
injections of residual magma Hke the "red rock" (soda 
granites, granitells), at the center which still remained at 
1400°. If this residual magma also consolidated at iioo^ C. 
it might take aplitic form, but if very rich in mineralizers, 
superheated steam, and so liable to consoUdate at lower tem- 
peratures, might be indefinitely coarse and pegmatitic. Such 
in fact appears to me to be the probable conditions for 
many dike rocks. 
Regarding the granites and gneisses called Laurentian 
and regarded by many as the softened base of the geological 
column, it is now pretty generally agreed that, as was clearly 
pointed out bv Lawson, they behave in their relation to such 
formations as the Keewatin exactly like any other igneous 
intrusive. ^Moreover their grain is not peculiarly coarse, 
though (and this has indeed been noted by Rominger and 
Lawson at least, as well as myself) the feldspar often be- 
comes gradually coarser at the margin, "porphyroidal" or 
"porphyritic," than farther from the contact. From these 
facts we may infer that there was a very considerable dif- 
ference in temperature (conditions of consolidation) between 
the Keweenawan and the "Laurentian" and also that the 
countrv rock was nearer the consolidation point of the feld- 
