CHAPTER I. 



SHRINKAGE FISSURES RECENT DEPOSITS. 



ALL rocks, no matter of what composition, are, in a 

 greater or less degree, traversed by cracks, joints, and 

 faults, and to explain the origin of these is the task 

 that first presents itself. In order to do this, we 

 must either make similar breaks artificially, or watch 

 what is going on at the present day. A recent de- 

 posit or accumulation, such as a bank of silt in a 

 river, lake, or estuary, or an alluvial flat un watered 

 by the lowering of a river, or a drained bog or moor, 

 when exposed to atmospheric influences, will crack as 

 it dries and contracts, sometimes irregularly (fig. 1, 

 PI. I), but often more or less regularly (fig. 2, PI. I.) 

 However, on account of the frail nature of the mate- 

 rials, rain, frost, heat, and the wind may more or 

 less obliterate the fissures, but some will remain open 

 to act as water-carriers if they have a fall ; they will 

 thereby be deepened, while the increase in their width 

 will probably be small. Numerous examples of such 



