68 JOURNAL OF GEOLOGY—SUPPLEMENT 



Sudbury sheet, in method of intrusion, great horizontal extension, 

 and stratiform arrangement of their differentiates, of which we see 

 only the more salic. If in any case, basaltic magma collected in a 

 chamber comparable in size with that occupied by the Sudbury 

 sheet, but of considerable vertical instead of horizontal extension, 

 it is unlikely that any amount of later disturbance or erosion to be 

 reasonably postulated would bring to light anything but the light 

 saUc differentiate. How much more true is this of bodies of magma 

 of very great dimensions! While, therefore, nothing in the fore- 

 going forces the conclusion that great granite bodies are merely the 

 light differentiate of a basaltic mass, nevertheless the conclusion 

 is entirely consistent with the observed tendency which accom- 

 panies increasing size. 



The features pointed out for the North American province 

 described are not local. They are world-wide in their occurrence. 

 Certain observations by Sederholm are pertinent in this connection. 

 Speaking of the fundamental rocks of Finland, he says that abyssal 

 rocks corresponding to almost every formation of effusive rocks of 

 the granite family are easily found in the deeply eroded complex, 

 but no abyssal rocks are found belonging to the diabase family, 

 though rocks of this latter family have been erupted during every 

 time of quiet sedimentation in Fenno-Scandia, either forming 

 effusive beds or being intercalated between the strata of the sedi- 

 ments. He considers that the erupted diabases represent basic 

 marginal phases of the common magma from which all the rocks 

 were derived and that they were drained off before the acid portions 

 because of their greater fluidity.^ It is necessary under this 

 assumption to imagine a marked differentiation of the magma 

 while still liquid, and an arrangement of the differentiates in 

 opposition to gravity. It is also necessary to imagine that in all 

 cases the basaltic marginal phase happened to be drained off. The 

 difficulties involved are entirely avoided if it is supposed that the 

 original magma was basaltic (diabasic) and that where intruded 

 in small bodies it formed, for the most part, sills and flows of 

 undifferentiated diabase or basalt. Where it collected in large 

 abyssal masses crystallization took place slowly. Crystals sank 



' Bull. Comm. Geol. de Finlande, XXIII (1907), 108. 



