66z 



THE POPULAR SCIENCE MONTHLY. 



step-like plains." Roads are carried up the Cordillera on elevated ter- 

 races to a height of 9,000 feet. 



These formations, so widely distributed and so uniform in their 

 aspects, have an important geological significance. They are evidently 

 among the latest results of the dynamic agents which have modified, 

 and are still modifying, the surface of the globe. Those along the 

 banks of rivers have been formed during the erosion of the valleys. 

 Their history, therefore, begins with the development of the present 



Fig. 1. 



Terraces in Limestone Cliffs, worn by 'Waves. 



river-systems, and comprises what is known in geoiogy as the "Ter- 

 race Epoch." They are most abundant and perfect in the drift lati- 

 tudes that is, where the continental floors are deeply covered by the 

 waste and debris of the Glacial Period, which closely preceded that of 

 the Terraces. If we examine the valley of a gently-flowing river, we 

 may study all the- processes by which it was formed, and step-like ter- 

 races distributed along its banks. 



There is the channel along which the stream is flowing. By the 

 side of it, at intervals, are verdure-covered meadows and deposits of 

 shingle and sand, overflowed during periods of rain and freshet. 

 These constitute what may be termed the river-flats or flood-plains. 

 Something is added to it during each overflow. Meanwhile, the river- 

 channel is deepening by the wearing action of the current and trans- 

 portation of the materials of its bed. At length the waters are dis- 

 charged along the channel, and no longer overflow the flood-plain, 

 which becomes at once a terrace, the last formed and newest of the 

 series, the oldest of which may be more than a thousand feet up the 

 bank. Fig. 2 shows a section of a river-valley with terraces on one 

 side only, a circumstance which frequently arises from sinuosities of 

 the stream. 



The newly-made terrace now really forms the bank or banks of the 



