STABLE AND MIGRATORY PLANT FORMATIONS 9 
drawn up in the folds ; and crystalline gneisses and plutonic 
rocks exposed by denudation of the mountain masses, being 
generally more resistant to erosion than unmetamorphosed 
sediments, contribute to their permanence. These move- 
ments also have a special tendency to recur at intervals over 
long periods of time, and the resulting mountains form major 
orographical features of the surface of the globe. On 
cessation of the movements, the mountains are gradually 
removed by denudation, and wide plateaux and plains of 
crystalline gneisses may form the sole geological evidence of 
their former existence. From time to time in geological 
history, new lines of weakness have developed with a new 
orientation, causing profound changes in geography, affecting 
the ocean currents, the climate, the distribution of rocks and 
their relations to erosion. 
2. From upward or downward displacements of the strand- 
line, either limited in extent and due to orogenic or to 
isostatic movements, or world-wide in their effects. The 
latter, according to Suess, are due to accumulation of sedi- 
ment on the sea-bottom, or subsidence of the ocean floor (8), 
and Penck has suggested that the formation of great ice-caps 
may affect the sea level the world over (9g). Such changes 
of level lead to diminution or expansion of continents, and 
alterations in their relations to islands and the areas subject 
to erosion, with changes in the barriers to plant migration, 
in the distribution of ocean currents, and in the range of 
humid, maritime, arid, and continental climates. Suess has 
suggested practically world-wide transgressions of the sea 
over the land in the Middle Devonian, Middle and Late 
Carboniferous, Early and Middle Jurassic, and Cretaceous 
times. On the other hand, during the Lower Devonian, 
Late Devonian, and Early Carboniferous, the New Red 
Sandstone and Late Jurassic, there were negative phases 
or downward displacements of the strand-line. There is 
good reason for believing that widely distributed geological 
deposits belonging to the negative phases are of continental 
and frequently of desert origin, and the known fossil fauna 
and the nature of the associated rocks point to life in and 
around inland seas and lakes. The continental areas of 
desert and small rainfall may have had a different relation 
