NORTH AMERICAN GEOLOGY. 207 



stone, quite distinct from the underlying coal-measure shales, and pos- 

 sibly unconformable to them. Of the one hundred and twenty three 

 coal- measure species in Nebraska, not more than a dozen pass upward 

 into this group. The overlying Dakota group lies on its irregular 

 eroded surface. It is thought that the old maps representing this 

 formation, extending from the Kansas line to the Platte, are erroneous, 

 as on the surface the group only appears to cover Gage and parts of 

 the adjoining counties. Newberry, in discussing this paper, questions 

 the propriety of calling the beds Permian, as their fauna is not charac- 

 teristic, and expresses the opinion that this formation does not occur in 

 America. Walcott stated that he regards some Arizona beds as good 

 representatives of the Permian. 



59. Cornwall Iron Mines, Pennsylvania. — So many opinions have been 

 held in regard to the position and origin of the ore beds at these im- 

 portant mines, and the similar ones northward, that the papers on the 

 subject which have appeared during the past year have more than usual 

 general interest. One is Willis's long-delayed report of studies made 

 in 1881 for the Tenth Census,* and the other is a very detailed report 

 by Lesley and d'Invilliers t of work in 1885. These writers agree in 

 considering the deposits entirely independent of the Mesozoic and to 

 have been derived from calcareous shales or limestone into which they 

 are found to graduate. Lesley and d'Invilliers, from a careful study of 

 the structural relations, consider the horizon of the ore to be approxi- 

 mately Trenton and apply the term " Cornwall Slates " to the extension 

 of the unaltered shales along the Trias border. Willis suggests the 

 possibility of the o;e representing the Upper Primal and that the lime- 

 stones are above ihe ore; their apparent subordination at some points 

 being due to cross faults, of which other evidence was found. Les- 

 ley thinks that the ore body is separated from the adjoining Trias by 

 a fault, but Willis and d'Invilliers do not share this opinion, consider- 

 ing the overlap to be that of a simple shore line. Lesley and d'Invil- 

 liers describe many details of the trap associated with the ore, and con- 

 sider its curved outcrop due to branching dikes and intrusions along 

 a slightly crumpled mouoclinal. 



CAMBRIAN AND TACONIC. 



The Taconic controversy is now rapidly approaching its end, and if 

 discoveries of fossils and determinations of structural relations continue 

 as in the past year or two, it will soon be satisfactorily terminated. 



The manner in which much of the literature on the subject has been 

 received indicates that in this, as well as other questions of its class, 

 nothing will be convincing to most geologists but the results of careful 



* Report on Mining Industries, pp. 223-234. 



t Second Geological Survey of Pennsylvania, Annual Report for 1885, pp. 461-470, 

 and pis., and by d'Invilliers, Am. lust. Mining Eng., Trans., 1886. 



