110 A MANUAL OF TOPOGRAPHIC METHODS. 



DEPOSITION FROM VOLCANIC ACTION. 



Deposits from volcanic action may be grouped as follows: (1) of liquid 

 lava, in the forms a, of streams and lakes, resulting in plains, tables, and 

 mesas, and b, of cones with craters, with gentle slopes. (2) of scoriae and 

 cinders, of which have been built cones with steep slopes, either with round 

 tops or with craters. 



Deposits of the first group consist largely of fields of basalt which have 

 been poured out from low vents or craters and spread in horizontal sheets, 

 in many cases covering great extents of territory. The Snake river plains 

 of Idaho furnish an example. As most of these eruptions are of recent date, 

 these sheets of basalt have suffered little from erosion, their form remaining 

 much the same as when they were poured out and spread over the land. 

 The surface is undulating, broken here and there by low cliffs marking the 

 edges of the flow, and by cracks and fissures here and there, especially near 

 the borders of the field. Owing to the frequency of the fissures, flowing 

 water is scarce upon these basalt fields, for the streams, sinking in the fissures, 

 find underground channels, to reappear at the borders of the fields in springs. 



AQUEOUS AGENCIES. 



The principal agency in shaping topographic forms is aqueous erosion. 

 In nine-tenths of the area of the United States the work of this agency is 

 prominent, while over much the larger part of the country the forms are 

 apparently due entirely to this action. It is so commonly seen, that the 

 topographer finds himself unconsciously reasoning in accordance with its 

 laws and attempting to apply them to forms produced by other agencies. 

 A country shaped by aqueous erosion is to him a "regular" country, while 

 one shaped by other agencies, less known, is irregular. The former can, to 

 some extent, be foreseen. In such a region, one reasons from the seen to 

 the unseen, while the vagaries of the latter can seldom be predicted. By 

 its agency the Appalachian mountains have been reduced from a compli- 

 cated system of mountain folds to the present comparatively low and simple 

 system of sandstone ridges and limestone vallej*s. In the Cumberland 



