LAKES AND VALLEVS OF NUGSUAK PENINSULA 665 
There are wide variations, however, from the steep sided and 
precipitous profile to that of considerably gentler slope. Frost 
action has wrought widespread results on the original glacial 
outlines of this form, resulting generally in a modification tend- 
ing toward a less precipitous outline. 
It has already been stated that numerous dikes of a basic 
eruptive rock, probably diabase, are found cutting the gneiss in 
an approximately vertical attitude. The basic dike rock of the 
peninsula has yielded more readily to the attack of the 
atmospheric agencies than the inclosing gneiss. Subsequent 
ice advance removed most of the residual decay, resulting in some 
instances, in the ice eroding considerably below the line of actual 
decay, determining thereby one set of the major valleys. The 
U-shaped outline is especially applicable to this type of valley. 
Similar conditions apparently prevailed at Turnavik, Labrador, 
as on the Nugsuak. The rock is a coarse porphyritic gneiss cut 
by dikes of a basic eruptive, many of which have been eroded 
to a depth of some 20 to 25 feet by the ice, and in some of which 
residual decay was still to be seen, indicating a greater preglacial 
decay of the dike than country rock. 
In the Umanak district of North Greenland, Professor Bar- 
ton,* observed that the dikes of eruptive rock had not been 
eroded below the level of the inclosing gneiss, but instead, 
they were found, in some instances, standing “out in relief above 
the oneiss:,” 
No petrographic study has been made of the rocks in the 
localities mentioned above, hence it would not be safe to say 
whether the difference could be accounted for on lithologic 
grounds, or whether it is due to a difference in glacial conditions. 
A second type of valley, but of minor importance, on account 
of slight development, was noted, the origin of which was trace- 
able to the etching out of the softer rock layers in the highly 
tilted strata by differential atmospheric weathering, and can 
therefore be classed as strike valleys. 
Subsequent to the period of widespread general glaciation, 
* BARTON, GEo. H., Technology Quarterly, 1897, 10, 236, 237. 
