1893.] NATURAL SCIENCES OF PHILADELPHIA. 333 



trickles for a short distance from pool to pool. The pools are gen- 

 erally rock walled but have not been formed in the manner of pot 

 holes. The original beginning of a pool may have been after the 

 manner of mechanically formed pot holes or may have been from 

 some chemical weakness of the rock. Some of these pools which are 

 many yards in extent and very irregular, are plainly the result of 

 corrosion chiefly, a process which is continually extending the area 

 of the pool since the water is constantly standing in it. Small 

 waterfalls of similar origin are quite prevalent ; but nothing shows 

 the effect of corrosion as well as the undercut terraces common in 

 the limestone of the Silurian, Carboniferous and Cretaceous. These 

 terraces, which are in the form of long caves parallel to the course 

 of the stream, mark the ordinary high water stage of the stream. 

 They bear a general resemblance to some sea caves and in some 

 cases are of mechanical origin where the high water stage coincides 

 with some lithologic weakness, as is often the case in the Cretaceous, 

 but, more frequently, and particularly in the Silurian and Carboni- 

 ferous, they are the direct result of corrosive action of flood water. 

 Sometimes one or two of these benches, somewhat destroyed, by sub- 

 serial denudation, exist on the bluffs above the one at present form- 

 ing, marking former high stages when the river bed was higher. In 

 these creek bluffs, from ten to fifty feet above the creek bed, may 

 also be seen actual caves extending far into the wall, marking the 

 former existence of underground streams or springs at a time when 

 the bed of the creek was much higher than at present. Springs, 

 perhaps similar to what these formerly were, are quite common in 

 the Silurian, as for instance, in Wallace Creek where several large 

 springs bubble up from the creek bed at the base of a high bluff of 

 Silurian magnesian marble. 



The streams of the sandstone region, of which Antelope Creek in 

 Mills County may be taken as a typical example, present the same 

 general features as those in the limestone ; but the walls are rarely 

 as steep and high and caves are seldom seen. This is the natural 

 result of the difference in lithologic character, and for the same rea- 

 son the pools are not of corrosive but of mechanical origin. In the 

 great supply of sediment produced, frequently from the denudation 

 of the Trinity conglomerate and sands of the lower Cretaceous, the 

 floods find more .material than can be carried away and the stream 

 bed is littered with bars and small deltas, which, during the greater 

 part of the year, form the enclosing sides of temporary pools, some- 



