834 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Minor interruptions indicating relatively local movements. — Besides 

 the submergences which define the major divisions of the stratified column, 

 there were very many minor, or incomplete, sea withdrawals. Perhaps 

 they should be called local, but I hesitate to do so because it is only the 

 criteria of sea retreat that vary in degree of development and distinctness 

 from place to place. The boundaries are nearly always there, though they 

 may be relatively inconspicuous in proportion to the locally diminished 

 gap in the record. These minor gaps are being more and more clearly 

 recognized in stratigraphic investigations, and as the geologist searches 

 them out and studies their relations to gaps in other sections their number 

 is being greatly increased. 



Detailed stratigraphic studies of this kind are being carried on with 

 striking success by European paleontologists engaged on Mesozoic rocks. ^® 

 In America we have not yet attained such refined results, though this is 

 due mainly to the absence or scarcity in most of our Mesozoic deposits of 

 fossils, like the semipelagic ammonoids, with sufficiently complex struc- 

 ture to permit the detection of slight mutations. Perhaps the highest 

 degree of refinement in correlation so far attempted in America is that in 

 an as yet unpublished work on the Ordovician rocks of the Cincinnati 

 geanticline. The principal results of these studies are outlined in suc- 

 ceeding pages of the present work (pages 416 and 526), where they arc 

 cited to illustrate simple and differential oscillations ("tilting'*) and 

 correlation by lithologic criteria. (See also Correlation Tables.) 



Somewhat different examples of overlapping formations are the Silu- 

 rian deposits in small narrow embayments on the west flank of the Nash- 



" since this was written S. S. Buckman has published an Interesting paper on "Cer- 

 tain Jurassic (Lias-Oolite) strata of South Dorset" (Quart. Jour. Geol. Soc. London, 

 ixvl, No. 261, Feb., 1910) that may be cited as an excellent illustration of advanced 

 modern methods in correlation. A few sentences quoted from this paper will bring out 

 the results of principal interest In this connection : 



"The deposits of one place correspond to the gaps of another. Therefore many locali- 

 ties have to be placed together to produce the full tale of the Inferior Oolite. The very 

 local distribution of Inferior Oolite species often means that strata of particular dates 

 have only been preserved in a few favored localities. . . . The beds of the 'Inferior 

 Oolite,' in a restricted sense, have now been divided as deposits of about twenty-two 

 successive dates or hemerse. The total for the whole of the Jurassic would not be more 

 than about eighty-five, or perhaps, on an extended scale, a hundred hemerse. 

 One can hardly view the few feet of Inferior Oolite limestone at Burton Bradstock. 

 about 15 to 20 feet say, and imagine that it represents an interval of time equal to a 

 quarter or a fifth of the whole .Turassic period — a time during which thousands of feet 

 of strata were laid down. But this Is because we do not allow suflBciently for the gaps. 

 . . . The Upper Lias part of the Junction Bed of Down Cliffs. Chideock (Lower or 

 pre-striatulus Toracian), is a very condensed, imperfect epitome in 20 Inches of about 

 180 feet of strata on the Yorkshire coast, and of very much more when allowing for 

 gaps. . . . Between the tifroni layer and the striatulus layer of the Junction Bed 

 there Is occasionally a 2-lnch layer which is all that represents some 250 feet of deposit 

 In the Cotteswolds — so that about two feet of .lunction Bed was formed while a thick- 

 ness of some 550 feet was being deposited elsewhere." 



