FACTORS INFLUENCING REPRODUCTION 829 



ing the habitats. The periods of the different outcrops have not been 

 determined with exactness, but are unnecessary for our purpose. It 

 appears that after long periods of deposition, diversified by volcanic 

 action, a large mass of granite was forced up into the strata. Whether 

 this granite ever reached the surface or not is unknown, for the highest 

 points now remaining were far beneath the surface and have been laid 

 bare by erosion. This granite was in turn penetrated by numerous 

 trap dikes, which also never reached the surface, so far as we know. 

 The granite now comprises practically all of the higher parts of the 

 island and extends to the sea. The rocks into which the granite was 

 forced are mainly metamorphosed quartzites and shales, with small 

 areas of lava (felsite) which seem to have come to the surface during 

 the periods of deposition before the intrusion of the granite. These 

 non-granitic rocks form the lower parts of the island, but are of con- 

 siderably less importance than the granite because they are largely 

 mantled with glacial till except along the steeper parts of the shore and 

 on low knolls on the northern part of the island. 



The geological events which have left their imprint most strongly on 

 the present habitat are the ice-invasion and subsequent submergence. 

 The ice polished and cut great gashes in the granite range which forms 

 the backbone of the island. It swept away the soil and in its place 

 left an incomplete mantle of stony till. 



The submergence following the ice-invasion removed the till from 

 all but the gentler slopes and level areas, leaving the granite hills al- 

 most bare of soil. On these hills only occasional small depressions and 

 the gentler northern slopes retain part of their covering of till. 



Geologists do not agree as to the extent of the submergence. Davis 

 admitting the 220-foot shoreline, but doubting the others. Shaler's 

 evidence is strong and corroborated by a number of stretches of shore- 

 line in addition to those he visited. Shaler claims that the submergence 

 covered the summit of the highest point (Green Mountain, 1,527 feet). 

 During the subsequent uplift of the land the sea did not stand long 

 enough at one level to cut in very deeply, except in two places, one at 

 about 480 feet and the other at 220 feet above its present level. Above 

 480 feet, wherever the slope is steep enough to have given the waves a 

 chance to cut, there are distinct terraces and cliffs, some of them small, 

 but almost certainly due to wave action. The geological history of the 

 island has therefore a direct bearing on the present vegetation. Al- 

 though the formations are old, the physiography is young and affords 

 many opportunities for studying pioneer associations. 



