12 proceedings: geological society 



than those which transpired thru evolutionary development during the 

 time of deposition of the Tombigbee sand and the overlying Selma 

 chalk and its non-chalky equivalents, or in terms of the Western Interior 

 section, approximately during the time required for the deposition of 

 the Niobrara formation and the Montana group. 



Harris and Vaughn have both pubHshed statements to the effect that 

 there is a great break, both stratigraphic and faunal, between the Cre- 

 taceous and the Eocene, as evidenced by the fact that not a single 

 species is kno^vn certainly to have crossed from one system to the 

 other. Vaughan characterized the differences as expressive of a com- 

 plete faunal revolution. In his opinion the changed that took place in 

 the marine life of the Atlantic and Gulf Coastal Plain during the time 

 represented by the unconformity, are more striking than those that have 

 taken place between earliest Midway time and the present, for no great, 

 orders that lived during Midway time, comparable to the Ammonoidea, 

 have become extinct. 



In the opinion of E. W. Berry, as expressed in a letter to the writer, 

 the differences in the Cretaceous and Eocene floras of the eastern Gulf 

 region are profound, and he believes that the unconformity represents 

 a very long interval. 



The hiatus marks a great diastrophic movement, as a result of which 

 the shore lines were pushed far to the eastward and southward. The 

 faunas were thus forced into new environments. Altho there is no evi- 

 dence of marked climatic changes, unknown factors may have induced 

 more rapid evolutionary changes in the marine life, but liberal allowance 

 being made for a quickening of development, the hiatus must have been 

 of great duration in order to produce the observed changes, even when 

 measured in terms of geologic .time. How much of that time should be 

 classed as Cretaceous and how much as Tertiary cannot be determined 

 with the available data. Cretaceous time, however, did not end with 

 the deposition of the uppermost Cretaceous strata as preserved, neither 

 did Eocene time ])egin with the deposition of the lowermost Eocene 

 strata, but the lines separating the two periods probabl}^ lies somewhere 

 toward the middle of the hiatus. 



The contact metamorphic copper deposits at Mackay, Idaho. J. B. 

 Umpleby. The contact metamorphic copper deposits at Mackay, Ida- 

 ho, are of particular interest because thej^ represent the replacement of 

 engulfed blocks of limestone after the solidification and jointing of the 

 porphyry inclosing them. Eight of these engulfed blocks and several 

 shoots of garnet-diopside rock, clearly of limestone derivation, crop out 

 in an area of about one scjuare mile. The most complete metamorphism 

 is of blocks along a pronounced fault, but metamorphism is locally in- 

 tense where onh' normal jointing of the porphyry was observed. 



Both exomorphism and endomorphism are clearly recorded. The 

 former changed the blue limestone of White Knob into white marljle 

 thruout a mass one square mile in area and 1000 feet thick. It also 

 caused a conspicuous marmorization of the engulfed blocks of limestone. 

 Here a broken zone of garnet-diopside rock is external to the marble 



