478 E. O. ULRICH CORRELATION OF THE STRAND-LINE 



would scarcely have been distinguished from their Mendota congeners. 

 Under the circumstances the specimens were very carefully prepared, and 

 then as carefully compared with collections from six different exposures 

 of the typical Mendota. The result of these comparisons shows that 

 whereas no structural differences whatever could be detected between the 

 specimens of the same species from the various Mendota localities, those 

 from the Black Earth dolomite proved in every instance to be distinguish- 

 able by inconspicuous, yet constant, differences. 



It is to be observed, however, that in both instances the specimens are 

 preserved as casts of the interior, and if these can be distinguished Ave 

 may believe that more perfect specimens would exhibit other perhaps 

 more important differences. But in their present condition the mol- 

 luscan remains from the two zones are distressingly similar. 



But it is not to be overlooked that these Mendota-like fossils in the 

 Black Earth dolomite are associated with three fossils that, so far as 

 known, have no representatives in the true Mendota fauna. These, then, 

 are for the present the real guide fossils for the Black Earth dolomite 

 zone. 



A HELDERBERGIAN INVASION OF THE ONONDAGA CORAL FAUNA 



The second instance concerns the presence of a coral fauna of 31 

 species in the midst of beds holding a normal late Helderbergian — New 

 Scotland and Becraft — fauna. The beds referred to occur at and in the 

 vicinity of Big Stone Gap, in southwest Virginia. They lie unconform- 

 ably on limestones of Cayugan age and are followed unconformably by 

 black shale of Genesee or Portage age. Some years ago Professor Schu- 

 chert determined their age to be Coeymans and New Scotland, his opin- 

 ion being based on the brachiopods contained in the small collections 

 made by him at Big Stone Gap. Somewhat different views were enter- 

 tained by Williams and Kindle about the same time, but they were not 

 sufficiently definite to require quoting here. 



In 1912, however, Doctor Kindle made larger collections, particularly 

 from one of the coral zones. Being impressed by the very striking 

 Onondaga aspect of the coral fauna, this author assigned the upper part 

 of the limestone unreservedly to the age of the Onondaga. The brachio- 

 pods and other fossils found beneath and in association with the corals 

 were not specially considered by Doctor Kindle. 



I became interested in the problem through the necessity of reporting 

 officially on the age of similar collections made by Federal Survey geol- 

 ogists engaged in the preparation of geological maps in adjacent areas. 



