THE SUDBURY LACCOLITHIC SHEET 773 
There are no ore bodies and hardly any smaller aggregations of ore in 
the norite above the basal edge; all have settled to the bottom. 
The gravitational segregation of rather acid norite from rather 
' basic micropegmatite seems to me on reflection not so certain, though 
the factor of time is in favor of it. The sediments above the eruptive 
sheet, without allowing for later erosion, formed a blanket 8,300 feet 
thick above the molten rock, and the vast mass of the sheet itself, 
more than 6,500 feet in thickness, must have conduced to excessively 
slow cooling. There are no certain means of estimating the original 
temperature nor the rate of cooling; but that the magma was very 
fluid when it spread out at first is shown by the way it has penetrated 
all the devious passages of the offsets described on a former page. 
During the hundreds of thousands of years required for cooling under 
the conditions mentioned there was time for even slow movements of 
segregation to accomplish great results. 
On the other hand, the theory of stoping should be considered. 
At the time the field-work was done in the Sudbury region my atten- 
tion had not been called to this mode of accounting for the micro- 
pegmatite, and no observations were made to test its applicability; 
but the analogous occurrences described by Dr. Daly in southern 
British Columbia, and his interpretation of Professor Bailey’s obser- 
vations at Pigeon Point on Lake Superior are very suggestive.t I 
have seen no direct evidence of stoping on a large scale from the over- 
lying conglomerate, but have little doubt that the process was to some 
extent a factor in producing the micropegmatite. The very unequal 
thickness of the Trout Lake conglomerate from point to point may 
perhaps be accounted for by unequal stoping. 
The peculiar features of the contact, where the boundary between 
the eruptive and the sediment vanishes, so that there is a real transi- 
tion between the two, seem better explained by penetration and pro- 
eressive solution of the conglomerate by the eruptive. It is quite 
possible, however, that at first much of the conglomerate was stoped 
away in blocks, sinking deep into the magma and becoming com- 
t American Journal of Science, Vol. XV (1903), pp. 269 ff.; “The Okanagan 
Composite Batholith,”’ Bulletin of the Geological Society of America, Vol. XVII, pp. 320 
ff.; ‘‘Differentiation of a Secondary Magma through Gravitative Adjustment, ”” 
Sonder-Abdruck aus der Festschrift zum siebzigsten Geburtstage von Harry Rosen 
busch. 
