STABLE AND MIGRATORY PLANT FORMATIONS i 
Should this view prove correct, it is probable that, as in the 
case of the lately glaciated districts, edaphic influences took 
a share in the evolution of the immigrating flora and the 
plant associations. 
4. From volcanic action sterilising wide areas. The early 
plant colonisations following such sterilisations have been 
described by Treub in the case of Krakatoa (12), and by 
Gronlund for certain lava-flows in Iceland (13). The results 
of vulcanism in various regions during the world’s history 
must have been stupendous. The late Tertiary volcanic 
eruptions of the Snake River plain in Idaho are estimated to 
cover an area greater than France and Great Britain com- 
bined, and still form a desert of sand and bare sheets of 
black basalt. These fissure eruptions have buried the topo- 
graphy under 2000 feet, and in some places 3700 feet, of lava, 
only the higher summits projecting above the volcanic floods. 
Fissure eruptions of the Cretaceous period in India now form 
the plateau of the Deccan traps over an area of at least 
2,000,000 square miles to a thickness in places of 6000 feet or 
more (14). 
5. We must also keep in mind the possible effects of the 
slow regional changes brought about by the agents of erosion. 
The degradation of mountains and the production of wide 
alluvial plains must, of course, be accompanied by profound 
alteration in the vegetation, from change of climate, from 
removal of barriers to plant migration, and from the trans- 
formation and translation of soils. The effects of these 
agencies under continued favourable climatic conditions 
would cause a progressive succession towards the mesophytic, 
as pointed out by Cowles. Where, however, from the 
change in geography, greater drought ensued, the vegetation 
might undergo retrogression, and may sometimes have cul- 
minated in desert conditions, with further destruction of the 
peneplain by wind erosion. Under humid climates, the 
continued processes of erosion and deposition on a stable 
crust gradually lead to the formation of deeper soils of a 
more and more mixed or uniform character. The rock 
exposures disappear, and the sorting power of the streams 
gradually diminishes as the base level of erosion is approached. 
Where, however, the climate becomes arid, erosion may 
