

58 STRUCTURAL AND FIELD GEOLOGY 
more or less angular, having travelled usually no great 
distance. 
Besides rain-wash and brick-earth there are various other 
products of the weathering of rocks, but these will be con- 
sidered later on when we come to discuss the nature and 
origin of soils and subsoils. 
Laterite is a red or brown, porous or cellular, ferruginous 
clay, common in India and other humid tropical countries. 
The ferruginous constituent may be diffused equally through 
the mass, or aggregated irregularly. Laterite is readily dug 
up, but becomes very hard when dried. It is the product of 
the subaérial decomposition of various rocks, such as gneiss, 
mica-schist, and other crystalline schists, diorite, basalt, and 
other eruptive rocks. 
Terra rossa is a red or brownish ferruginous earth met 
with more or less abundantly in regions composed of limestone 
and calcareous rocks. It is simply the insoluble residue 
derived from the dissolution of these rocks by atmospheric 
action. It assumes a great development in the limestone 
regions of southern Europe, but may occur wherever such 
rocks are exposed to the action of the weather. The red 
earth, so frequently met with in limestone caves, is of the same 
origin, and has been introduced for the most part by rain and 
melting snow, through fissures communicating with the 
surface. 
Blown Sand and Dust.— Blown Sand accumulates under 
all conditions of climate—wherever, indeed, loose sand is 
exposed to deflation or the transporting action of the wind. 
Hence dunes and sheets of wind-blown sand are well developed 
upon certain sea-coasts and lake-shores, and in the broad, flat 
valleys of many large rivers. In such regions, however, the 
wind acts chiefly as a transporter of disintegrated rock- 
material—the sand having already been prepared for it by 
the action of other superficial agents, such as tidal currents, 
waves, rivers, etc. In dry, desert tracts, however, blown sands 
owe their origin and distribution mainly to the combined 
action of insolation and deflation. By alternate expansion 
and contraction rock-surfaces are broken up and comminuted, 
and the grit and sand thus formed are carried forward by the 
wind, This loose material, swept against upstanding rocks, 

