DISLOCATION OF STRATA. 101 



lies on upturned older rocks, is more or less faulted, and is overlaid 

 by horizontal alluvial beds. 



In such cases of unconformability, the upturning of the lower beds 

 must have taken place 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. 



When, after the deposition of beds in a continental sea, or along its 

 borders, a sinking of the region takes place, the next deposits there 

 made would extend beyond the limits of the preceding, and overlap 

 on those outside. In such cases, although both deposits are approxi- 

 mately horizontal, there is still a kind of unconformability called an 

 overlap. 



When strata are faulted, there may be perfect conformity of dip between the beds 

 either side of the fault, as in Figs. 110 and 111 C, and yet no conformability, since this 

 relates to supeiposition. So there may be unconformity as to dip on two sides of a 

 fault without unconformability. It is easy to be led astray by such appearances of un- 

 conformability, especially in regions of metamorphic rocks. Actual superposition must 

 be seen, before the fact of unconformability can be safely asserted. 



Deposits like those at e f are true basin — or trough — deposits ; for the}' - are 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 synclinal valleys 

 are shallow, it is easy, and not uncommon, 

 to mistake beds conformable with the 

 strata below for such basin-formations. 

 The beds a b (Fig. 113^ lie in the synclinal 

 valley m n, like a basin-deposit, though 



not so. They were formed before the folding of the beds, and not after it, — an his- 

 torical fact to be determined in all such cases with great care. 



4. Order of arrangement of Strata. 



The true order of arrangement of strata is the order in which they 

 were made, or their chronological order. All strata of the same era, as 

 nearly as can be ascertained, are said to be equivalent strata, or those 

 of the same geological horizon. 



As geological eras, even the shorter divisions, have in general been of very long dura- 

 tion, the equivalent strata of distant regions cannot be known to be precisely synchro- 

 nous in origin. A long time, measured by thousands of years, may in fact have inter- 

 vened between the commencement of beds that are most alike in all those points by 

 which we determine age and equivalency. 



Huxley, in view of the impossibility of determining true synchronism, has proposed 

 to designate by the term homotaxial (from the Greek 6/uds, same, and Tafi?, order) those 

 strata, in regions more or less widely separated, that have apparently the same relative 

 position in the geological series. 



Difficulties. — The following are some of the difficulties encoun- 

 tered in the attempt to make out a chronological order: — 



The stratified rocks of the globe include an indefinite number of 



