36 



DYNAMICAL GEOLOGY. 



as it runs over the sand-Hat, is often choked with its own 

 deposit, and compelled to seek new channels by dividing. 

 In the figure we have represented the case of a torrent, 

 carrying coarse sediment, rushing down a steep slope into 

 a lake or pond. In such a case the sediment falls quickly, 

 the strata will be irregular and highly inclined ; but, in 

 the case of great rivers carrying sediments for long dis- 



FIG. 15. Ideal map (a) and section (6), showing the formation of a delta. 



tances, the coarse material is all dropped higher up, and 

 that which reaches the sea is very fine, and therefore sinks 

 slowly. Hence in great deltas the stratification is nearly 

 horizontal. 



Again, the figure plainly shows that, if this process 

 goes on, the lake or pond will be entirely filled. All 

 mountain-lakes are being rapidly filled in this way, and a 

 little close observation is sufficient to show that all high 

 mountain-regions, like the Sierra or Colorado mountains, 

 are full of marshes and meadows which are extinct lakes. 



Age of Deltas. All deltas are growing. The rate of 

 growth in some cases has been observed. The delta of 

 the Po has advanced twenty miles into the Adriatic Sea 

 since Roman times, for the town of Adria, then a sea- 

 port, is now twenty miles inland. The delta of the Rhone 

 has grown thirteen miles in the Christian era. The Mis- 



