22 Schuchert and Barrell — Revised Geologic 



magmas, fluid at lower temperatures and more thinly fluid than 

 those which are richer in silica. The common solidified forms 

 are the basaltic surface rocks and, at greater depths, the 

 diabases and gabbros ; rocks rich in hornblende, pyroxene, and 

 olivine. On the other hand, the alkaline oxides, soda and 

 potash, tend to separate from the heavier metallic oxides and 

 keep with them most of the silica, the acid radicle of the 

 common rock minerals. They thus give rise to the acidic mag- 

 mas, solidifying into rocks dominated by alkaline feldspar and 

 quartz. Such rocks are, however, difficult to melt and become 

 pasty rather than fluid, when heated in the furnace. But in the 

 laboratory of nature the fusion takes place at great depths in 

 the crust and the magmas are surcharged with gases which, 

 because of the pressure of the overlying rock, cannot escape. 

 Their presence in various degrees of concentration gives all 

 degrees of fluidity and consequent capacity for intrusion. As 

 a whole, however, the basic magmas are probably more fluid 

 even at depth, and are markedly more fluid on approach to the 

 surface. The acid magmas at the surface betray their viscous 

 nature in pumice, obsidian, and rhyolite ; at depth, on the con- 

 trary, they give rise to granites and to the slightly more basic 

 forms known as granodiorites. In thin injection sheets and in 

 the vein-like nature of pegmatite dikes they there betray a high 

 local fluidity, but in the greater masses the structural phenomena 

 not uncommonly suggest a higher viscosity and therefore a 

 lessened capacity to rise through overlying rocks. 



The maintained fluidity of basic magmas makes easy their 

 extrusion in enormous volume, in spite of their high density. 

 In some regions the great lava fields of Cenozoic date show no 

 evidence as to whether more acidic phases of the regional mag- 

 mas are concealed beneath, but where erosion in the Cordillera 

 has exposed the granite sand granodiorites to view, the latter 

 are commonly seen to have been preceded in the same regions 

 by great extrusions of lavas and breccias, mostly of intermediate 

 or basic nature. A period of great batholithic invasion may 

 therefore be connected genetically with a preceding period of 

 lava and breccia outpourings. The Neolaurentian batholithic 

 invasion was preceded by very basic lava flows and tuffs, as is seen 

 in the upper parts of the Sudbury series. The Paleolaurentian 

 invasion was preceded by the vast basaltic flows of the Keewatin. 



These facts concerning the recurrence here and there through 

 geologic time of regional igneous activity, volcanic phenomena 

 culminating in subcrustal foundering and the cradling of the 

 surface rocks upon newer foundations, when taken in conjunc- 

 tion with the widely separated ages of the Paleo- and Neolauren- 

 tian invasions, show that it is no longer safe to regard the 

 Keewatin and Laurentian as parts of a primal igneous eon. 



