TERRANES. 



115 



in regular succession, each, stratum conformable in bedding to the preceding. 

 This was true of the 40,000 feet of rock of the Appalachian region (referred 

 to on page 353), out of which the Appalachian Mountains were finally made. 

 This is an example of conformability, as the term is used in geology. 

 Through the long series there is conformity in bedding. 



But these conformable strata rest on older rocks that have the bedding 

 upturned and standing at various angles. Between the two there is uncon- 

 formability in bedding. 



Fig. 130 illustrates this subject. The beds 2, 3, 4a, 4&, are conformable 

 to one another, but unconformable to the flexed rocks numbered 1. The 



130. 



1, Upturned Archaean rocks; 2, 3, 4o, 46, overlying strata, conformable with one another, but unconformable 



with the Archaean. Logan. 



flexing of the rocks antedated the deposition of No. 2 ; and knowing the 

 geological age of ISTo. 2, some approximation is made toward a knowledge of 

 the time of flexure. There may be three or four cases of unconf or inability 

 in the same region. For in each mountain-making epoch, new rocks are 

 upturned, and the succeeding ones are laid down horizontal, as usual, over 

 the upturned. Such unconformabilities belong especially to regions of moun- 

 tain-making; for there occur the upturned rocks. Only a few miles away 

 from the region of the mountain, the rocks that are unconformable in the 

 latter may rest on one another in regular order, or conformably, as if no 

 disturbance had anywhere taken place. 



The preceding figure has a fault-plane at /, and there is an unconformity 

 between the beds on each side of it, but not unconformability. The uncon- 

 formity introduced by faults is easily mistaken for true unconformability. 

 Such unconformity is of frequent occurrence in all formations ; while unco7i- 

 formity in bedding indicates an epoch of mountain-making, a thing of rare 

 occurrence in the geological history of a region. 



Besides this most important species of unconformability, that of t\vQ first 

 kind, there are also two other kinds: (1) through changed sea-limit or 

 overlap ; (2) through surface erosion. 



Tlirough overlap. — When, after the deposition of beds, a slight sinking 

 of the region takes place, the next deposits there made may extend beyond 

 the limits of the preceding, and overlap those outside. In such cases, 

 although both deposits are approximately horizontal, there is still a degree 

 of unconformability. Oscillations of the land surface, or of the water level, 

 have gone on through the successive periods, so that unconformity by overlap 

 is of very frequent occurrence, and of minor significance, though always of 

 great geological interest. 



