234: A. P. COLEMAN—CLASTIC HURONIAN ROCKS. 
Suess thinks that eruptives of this sort cannot elevate the rocks above, 
but, on the contrary, can only occupy spaces already prepared by ten- 
sions in the earth’s crust. * 
Some of the methods referred to are obviously inapplicable to the 
batholitic type of mountains. Russell’s suggestion of upwelling lavas 
urged by a squeezing action of the earth’s crust can hardly be brought 
to bear on a region where the whole solid crust for thousands of square 
miles has been‘tossed into irregular domes. In fact it is difficult to see 
how any outside force can be applied in such a way as to elevate domes 
50 miles across when the earth’s crust adjoining is itself plastic. We 
seem forced to look fer some force inherent in the masses themselves. . 
If we look at the conditions we find that the granites of these batholites 
were probably fused hydrothermally, but not excessively hot, since 
blocks of basic Huronian rock, readily fused by a dry heat too low to 
melt granite, often le in them with unrounded edges. Even at a rela- 
tively low fusion point they must have been much hotter and hence 
(potentially) lighter than the unfused rock above, particularly when the 
latter was basic like most members of the Keewatin series. This rela- 
tively light siliclous magma, probably not thoroughly liquid, but only 
plastic, following the laws of hydrostatics crept upward where the load 
of overlying rock was smallest, the heavier Huronian beds meanwhile 
settling slowly into synclines between the rising batholites. The process 
may be conceived to have gone on very slowly under sufficient load to 
prevent violent disruptions of the overlying strata, since a certain plas- 
ticity of the beds is shown by the shearing observed, especially among 
the softer pebbles of the conglomerates. 
The large porphyritic felspars observed in many of the gneisses and 
eranites suggest two stages in the history of these rocks—an earlier one, 
before the ascent began, and the later slow consolidation. Augengneiss 
around the margin of batholites proves that the crystals existed before 
the shearing uprise was complete. 
Some of the laccolites described by Gilbert show surprisingly perfect 
unbroken domes of stratified rock, and the same is true of the domes 
elevated by plutonic plugs; in both cases due, as suggested by Gilbert, 
to there having been load enough to prevent disruption. The amount 
of stretching undergone by the arched strata in the instances described 
was, however, not very great. 
In the case of the larger batholitic domes of western Ontario the ex- 
tension must have been as a rule much greater. The Grand Presquile 
dome may have been comparatively low and flat, since a dip of only 24 
* Antlitz der Erde, vol. 1, p. 218, ete. 
