STRATIGRAPHIC TAXONOMY 578 



cially referred to here, namely, the first between the Cambrian and the 

 Ozarkian, the second between the Ozarkian and the Canadian. The dis- 

 tinctness of these three systems probably would not be so confidently 

 asserted were it not for the belief that the orogenic movements at these 

 times were confined to belts so near the margins of the continent that 

 detrital material could not be supplied to the inland basins in which the 

 rocks of these periods are now exposed. Apparently for like reasons the 

 Siluro-Devonian transition in America is similarly devoid of clastic 

 deposits. 



Cardinal principle of the new stratigraphy. — The point that should be 

 continually borne in mind is that the accessible stratigraphic sequence, 

 at whatever locality and however obscure the breaks, is always incom- 

 plete. The more complete the stratigraphic record the more numerous 

 the hiatuses, the fewer the breaks the greater their average time values; 

 and the relative conspicuity of the stratigraphic break is never a safe 

 indication of its importance. A sandstone may be followed without 

 intervention of other deposits by another sandstone, or it may be tAvo 

 shales or two limestones that are in such close contact tliat continuous 

 deposition is suggested; and yet a great time break with emergence may 

 often be proved to have separated them. (See pages 536 to 532.) 



Neither is the relative volume of deposits nor their areal extent a safe 

 criterion. Two or three periods may be locally represented in a few feet 

 of sediments. This is really a common occurrence on the flanks of the 

 Nashville dome, where the Silurian and Devonian are often represented 

 by thin overlap wedges. On the flanks of the Ozark dome, indeed, not 

 only the Silurian and Devonian but the Ordovician and Canadian 

 systems as well are locally absent and never strongly represented. Here, 

 then, four periods may be represented in a varying sequence of deposits 

 aggregating but a few hundred feet in thickness. If these fragmentary 

 records were all that Avas known of these systems, it would be very 

 difficult if not impossible to prove their true significance in the time 

 scale. Fortunately, each is represented elsewhere by thousands of feet 

 of sediments. More fortunately still, we can prove by satisfactory evi- 

 dence that the thin wedges represent only small parts — usually the up]jer 

 part — of widely spreading formations of their respective periods, which 

 happened to extend by overlap to accessible situations on these flanks from 

 some other area in which a more complete record was laid down. 



Of all the features of the new conception, the smallness of the con- 

 tinental seas and the frequency and rhythmic occurrence of the oscilla- 

 tions, which caused withdrawal and shifting of the seas from one basin 



