112 LITHOLOGICAL GEOLOGY. 



formable to those beds. Below ef there are really two sets of un- 

 conformable beds in a synclinal valley; and, moreover, the lower 

 strata were much faulted and upturned before the upper were laid 

 down upon them. The Connecticut Eiver sandstone, like the 

 latter, lies in a synclinal valley of older rocks, is more or less faulted, 

 and is overlaid by horizontal alluvial beds. 



There is here exemplified a method of arriving at the period or time of 

 an uplift. In such cases of unconformability, the upturning of the lower 

 beds must have taken place after they were made, and before the 

 deposition of the overlying beds. The time of the upturning, therefore, 

 was between the period to which the upturned rocks belong, and 

 that of the overlying deposits. 



Deposits like those at e/are true basin or trough deposits ; for they are formed 

 in basins or depressions of the surface. Such deposits may, in general, be dis- 

 tinguished by their thinning out towards the sides of the basin. Yet when 

 synclinal valleys are shallow it is easy, and not uncommon, to mistake beds 

 conformable with the strata below 



for such basin-formations. The beds Fig. 113. 



m 

 a b (fig. 113) lie in the synclinal val- /£f3v 



ley m n like a basin-deposit, though ^b^^o^l^^^^s^fe^ ^^~L 



not so. They were disconnected ', "--fo^liSS^^^^^^^m^CT 



from the stratum with which they L ^^ii^ ^^^iS^3^^i 



were once continuous, by denudation 



over the anticlinal axes m and n. Hence the beds a b were formed before the 



folding of the beds, and not after it, — an historical fact to be determined in all 



such cases with great care. 



4. Order of arrangement of Strata. 



122. The true order of arrangement of strata is the order in 

 which they were made, or their chronological order. 



Difficulties. — There are several difficulties encountered in the 

 attempt to make out such an order. The stratified rocks of the 

 globe include an indefinite number of limestones, sandstones, 

 shales, and conglomerates ; and they occur horizontal and dis- 

 placed, conformable and unconformable, part in America and part 

 in Europe, Asia, and Australia, here and there coming to view, 

 but over wide areas buried beneath soil and forests. 



Moreover, even the same bed often changes its character from 

 a sandstone to a shale, or from a shale to a limestone or a con- 

 glomerate, or again to a sandstone, within a few scores of miles, 

 or, if it retains a uniform composition, it changes its color so as 

 not to be recognized by the mere appearance. Again, some 

 strata are of very limited extent, while others spread widely over a 

 continent. 



