116 



STRUCTURAL GEOLOGY. 



Through an interim of erosion. — Between the time of making two suc- 

 cessive horizontal strata there is sometimes an interval of exposure to 

 marine or fluvial erosion, which the worn upper surface of the lower stratum 

 indicates. This, also, is unconformability in geology, and as the interim of 

 erosion may be long, it is of importance. Yet in all periods, as in that 

 of existing time, the deposits made during a period may be extensively 

 worn away in some large regions before the period has closed ; partly worn 

 away in many places it is sure to be. An uplift of 600 feet in the present 

 era, putting a coral reef rock this much above the sea, is followed by cave- 

 making and extensive removals. The amount of erosion is no certain 

 evidence as to the length of time during its progress. 



Deposits are sometimes formed in basins or depressions of the surface. 

 Such deposits may, in general, be distinguished by their thinning out toward 



the sides of the basin. Yet, when syn- 

 clinal valleys are shallow, it is easy, and 

 not uncommon, to mistake beds that are 

 conformable with the strata below for such 

 basin formations. The beds ah (Fig. 131) 

 lie in the synclinal valley mn, like a basin 

 deposit; but they were formed before the folding of the beds, and not 

 after it. 



131. 



Unstratified Terranes. 



The unstratified terranes comprise (1) the great unstratified masses of 

 granite and other related crystalline rocks ; (2) the various masses of ejected 

 igneous rocks that lie in piles, not having the bedding due to su.ccessive 

 flows, and not making part of any stratified series; (3) masses occupying 

 fissures in the earth's crust or supercrust, and having thereby the nature 

 either of dikes or veins. 



The facts connected with unstratified terranes are necessarily considered 

 in Part III. on Dynamical Geology, and remarks here are therefore 

 unnecessary. 



