306 BOTANICAL GAZETTE [MAY 
IV; (fHE- ROCK. SOCIETIES: 
The pre-Cambrian rocks of the Marquette region furnish an 
excellent field for the study of the succession of plant societies. 
2.—Jack pine forest on a fossil beach 
near Manes e. The — of undergrowth 
is due to repeated surface fire 

The rocks are mostly gran- 
ites and quartzites. The 
chemical nature of the rocks 
seems to have little influ- 
ence on the ultimate plant 
society that is obtained, but 
the ease with which they 
disintegrate is different, 
therefore the rapidity of the 
succession of plant growth 
is influenced. Other things 
being equal, granites, be- 
cause of their heterogene- 
ous structure and conse- 
quently differential weath- 
ering, will furnish a soil 
more quickly than the 
homogeneous quartzite. In 
the region under discussion 
the rocks have not only 
been worn and_ polished 
smooth by the action of 
the last ice sheet, but since 
then have lain beneath the 
waters of the former exten- 
sion of Lake Superior. In- 
deed, some of them have 
only recently emerged from 
the lake. Thus just as 
there are beach lines of dif- 
ferent ages, so there are 
rock areas of different ages. 
Other things being equal, 
