808 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



come to be given the value of a geologic period. It is to be expected, there- 

 fore, that diastems occur here in rhythms of smaller and larger time 

 values. In the photography A constitutes the Fairmount, B and C the 

 Bellevue. TJlrich and Bassler consider that there is a well marked break 

 bet\¥een A and B. Bassler states in a personal communication that: 



"Between B and C strata come in, away from Cincinnati, sometimes to a 

 rliiclvness of 50 feet, whicli are represented around Cincinnati only occasionally 

 l)y an inch or so. These are the Orthorhynchula beds which are so well devel- 

 oped in central Tennessee. This is only one of the unconformities which have 

 been noted in the Cincinnati section where hitherto the rocks have been sup- 

 posed to record continuous deposition." 



In A there is a very marked rhythm which is not present in B or C. 

 It would seem, therefore, that the disappearance of the rhythm indicates 

 a long time interval and is in accordance with the evidence given by 

 Bassler. The difference between B and C may indicate a lesser diastem. 

 The significance of a sudden change of rhythm is the feature of this sec- 

 tion which is most noteworthy. 



The next illustration, plate 45, figure 2, is from the lov^er part of the 

 Silurian. The sequence is given as follows by Williams: 



F. Lockport dolomite, thin bedded. 



E. Eochester shale. 



D. Irondequoit limestone, thick bedded, top of Clinton. 



C. Walcott limestone, thin bedded, Clinton. 



B. Medina sandstone, thin bedded. 



A. Cataract sandstone and shale, Schuchert, thin bedded. 

 Schuchert states that there is no clear faunal break here. This organic 



evidence of a disconformity is therefore absent, but the great lapse of 

 time and the many sharp changes in sedimentation may be taken as evi- 

 dence of the existence of diastems of large time value, approaching the 

 nature of disconformities. The absence of faunal breaks would appear 

 to be a proper basis for classifying the hiatuses as diastems. 



In plate 46, figure 1, is shown a section which brings into one view for- 

 mations of widely differing ages. The formations represented are given 

 by Schuchert as follows: 



E. Chattanooga black shale, Mississippian. 



D. Onondaga limestone, 6 feet. Middle Devonian. 



C. Louisville limestone. Middle Silurian. 



B. Waldron shale, 9-10 feet. Middle Silurian. 

 A. Laurel limestone, Middle Siluriaii. 



Willis summarizes the statements of Foerstc; Kindle, and Barnctt in 

 regard to these Silurian formations of the Niagaran epoch as follows: 



