PRELIMINARY REMARKS. 
35 
erto been made. But the finest of the sediment probably passes 
over the river bar, and may be carried far from the delta by 
oceanic currents. The actual time, then, consumed in the 
formation of the delta is greater than the calculated. 
What takes place by the instrumentality of the Mississippi, 
takes place in the same mode by all.rivers. As the winds are 
the carriers of water, so the rivers are the carriers of sedi¬ 
ments. By the combined machinery of wind and water all the 
sedimentary rocks are formed. Water, acting upon these 
plastic materials, spreads them evenly upon deltas and over 
wide areas upon the ocean’s bottom. Here, subjected to a great 
pressure, they become consolidated into rock. 
§ 30. Winds, as the distributive agents of heat and moisture. 
The geological agency of the wind is modified by its direction. 
The trade wind of the Gulf furnishes a supply of water for 
the western slope of the Appalachian chain, and the northeast 
wind of the Atlantic brings a supply for the New England and 
Middle states. The mountains of Oregon condense the moisture 
of the northwest winds which have passed over the Pacific. 
To the Eastern states the same wind is dry and cold, having 
been robbed of its moisture, as well as cooled, by the highlands 
over which it has passed. A continent is indebted to the 
agency of winds for the supply of water, both for its vegeta¬ 
tion and that which is required to feed its rivers and streams. 
They dry the earth’s surface when wet. Sixty per cent of the 
rain wdiich falls in the valley of the Ohio, is restored directly 
to the atmosphere, or is taken up by vegetables. The south¬ 
western winds are warm and damp from having passed over the 
Mexican gulf. This great body of water is of the utmost 
importance to the well being of the Appalachian slopes and 
valleys, imparting moisture and a subdued temperature where 
in its absence it would be dry and hot. 
§ 31. The earth’s surface is acted upon mainly, as 1 have 
already stated, by two distinct and diverse agencies, fire and 
water: the former, by its well known properties, which are 
manifested in the simple expansion and fusion of matter; the 
