TRANSACTIONS OF SECTION E. 551 
ons impossible; the colonisation of America 
of them under primitive conditi : 
had thus to wait for the development of mechanical means of transportation 
upon land. : ; 
The chief economic fact in the history of the American continent is the 
railway development as a means of colonisation. 
The author described in general terms the situation of the more important 
natural resources, and gave a brief account of the localities of different 
industries. 
9. The Seychelles. By J. SvaNuey GARDINER, F.R.S.—See Fourth 
‘ Report on Investigalions in the Indian Ocean, p. 198. 
3. The Cycle of Alpine Glaciation. 
By Professor Wiun14M Hursert Hosss. 
Though differing widely in the nature of the erosional process and 
resulting in forms of wholly different aspect, there may be recognised in 
the sculpturing of upland regions by rolling glaciers a differentiation into 
stages analogous in some respects to those well determined for river erosion. 
Studies lately completed in many mountain districts once occupied by 
glaciers of the Alpine type, and the new and much-improved topographic 
maps, permit of new generalisation concerning relative degrees of maturity 
and the natural limitation of the cycle. 
4. The Formation of Arroyos in Adobe-filled Valleys in the South- 
Western United States. By Professor Ricuarp E. Dopce. 
Many of the valleys in Arizona, New Mexico, and other arid regions in the 
United States are partially filled with a compact clay of probable sub-aerial 
origin, known as adcbe. Such valleys usually have a longitudinal slope of 
not more than five degrees and a cross slope of somewhat more than five 
degrees. Many of these filled valleys are now being actively dissected and 
longitudinal arroyos from 20-50 feet in depth have been cut. These arroyos 
have few side branches and are being extended headwards mainly. The 
arroyos are evidently of recent origin, and a comparison of the accounts made 
by early expeditions in the Chaco Valley shows a rapid increase in length 
since the first reconnaissance surveys in the region. 
Evidence from the study of sections and peoples is strong that formerly 
the valleys were grass-covered and that the water after rains flowed over the 
gently sloping valley floors in the form of sheets. The reason for the present 
concentration of water and the consequent inauguration of valley-cutting is 
therefore an interesting question. The Navajos, the native race of the region, 
connect the change with the bringing-in of sheep by white men and the conse- 
quent close cropping of the native grass, coupled also with the fact that sheep 
travel in droves, often in single file. Thus for two reasons the sheep would 
aid in loosening the surface detritus and giving the water an opportunity 
to become concentrated in streams. That this suggestion is a strong working 
hypothesis is shown by the facts to be observed in the Chin Lee Valley in 
Arizona. While the main valley and its larger tributaries are barren and 
eut into by arroyos, one branch is grass-covered and flat-floored and supports 
willow trees. Water flows in sheets over the surface and is not concentrated 
in streams. This valley has never been occupied as a grazing area for sheep 
by the Navajos because of a superstition that it contains certain plants that 
are injurious to sheep. The suggestion that the inauguration of arroyo- 
cutting is due to the removal of the grass cover is strengthened by the fact 
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