482 FRANK F. GROUT 
3. The texture of an intrusive mass in relation to its borders 
and its contact metamorphism may be a sign of convection. Lane’ 
and Queneau’ have shown that in the case of simple conduction 
of heat away from a stationary mass the grain of the rock will not 
be uniform up to the contact unless the magma temperature was 
farther above the temperature of crystallization than the wall-rock 
temperature was below. As Lane puts it, measured from the 
temperature of the surrounding rock as zero, the magma must 
be more than twice the temperature of solidification. For an 
intrusion at moderate depths into cold rocks, this means more 
superheat than is commonly supposed to exist. The observed 
superheat is seldom over 200° to 300°C. 
4. The arrangement of the extreme differentiates in a laccolith 
might be a strong indication of convection. If the outer layer is 
of average composition it may be attributed to chilling, but if a 
layer found on all sides, top and bottom, is an extreme of the series 
of differentiates it can be attributed to neither chilling nor gravity. 
An illustration is found in the Lugar sill in Scotland.’ The average 
material of the sill has scarcely 20 per cent of salic constituents, but 
the border phases have over 4o per cent of feldspar and feldspathoid 
material. Since diffusion is shown to be too slow a process,’ the 
only way to get the extreme product segregated to all sides is by a 
circulation of some sort. 
5. A differentiated dike found by the writer at Duluth is very 
suggestive. The dike is four feet wide and is pegmatitic in nature 
a few feet below the base of the main gabbro. Its texture is particu- 
larly coarse at the sides, where the composition is that of a gabbro, 
and it is evident that the crystals must have grown large by addi- 
tions from the residual circulating or passing magma in the center 
of the dike. From these coarse gabbro borders there is a complete 
gradation toa medium-grained red granite in the center of the dike. 
t A.C. Lane, ‘‘The Grain of Rocks,” Bull. Geol. Soc. Am., VIII, 403. 
2A. L. Queneau, School of Mines Quarterly, XXIII (1902), 181. 
3G. W. Tyrrell, ‘Alkaline Igneous Rocks of West Scotland,” Geol. Mag., IX 
(1912), 75-77- 
4N. L. Bowen, ‘‘Later Stages of Evolution of Igneous Rocks,” Jowr. Geol., 
Supplement, December, 1915, p. 12. 
