366 J. E. TODD — CONCRETIONS AND THEIR GEOLOGICAL EFFECTS 



bank abounding in concretions will soon have a quantity of them inter- 

 fering with the further action of its waters. The same may be true also 

 where lakes and the ocean cut against deposits of similar character. 

 Hence concretions have an important effect in limiting the size of lakes 

 and the encroachment of the sea, and they also have much to do with 

 the features of river valleys. A stream cutting through rocky strata of 

 uniform hardness naturally produces canyons bounded by cliffs. In 

 softer strata of uniform character similar features may appear at first, 

 but very soon give way to gentle slopes. In other words, the stream 

 reaches grade easily and swiftly. The effect of concretions is interme- 

 diate between these two forms. The upper part of the slopes may cut 

 back to a gentle angle, and yet the stream becomes limited to a more 

 definite channel than in the second case, so that the bottom is not 

 widened, although the valley may be open. Moreover, the stream is 

 not likely to ever have much of a flood plain. Where streams pass 

 through a stratum of concretion-bearing clays with similar soft deposits 

 barren of concretions above and below, there is apt to be a narrower 

 v^alley and a steeper slope in the concretion-bearing region. 



These are some of the more obvious results of a study which, if con- 

 tinued with diligence and discrimination, promises to reveal still more 

 impressively the influence of concretionary action upon physiography 

 and the general history of the earth's surface. 



Explanation of Plates 



Plate 49. — Concretions, their Structure and Effects 



Figure 1. — A secretion. 



Figure 2. — A geode. 



Figure 3. — An intercretion, showing stages of growth by sections through one of 

 globular form. 



Figure 4. — Sections through a lenticular intercretion. a, vertical; 6, horizontal. 

 When the cracks are filled with mineral it becomes a septarium. 



Figure 5. — Section through a compound intercretion like numbers 17 and 22, 

 plate 50. 



Figure 6. — An accretion with nucleus. 



Figure 7. — A cavern, showing stalactites, a, formed by trickling water, and b, by • 

 seeping water, and c, stalagmite, all accretionary growths but not 

 concretions. 



Figure 8. — Theoretical diagram of an isometric molecule, showing axes or bonds. 



