282 



The Artinskian of Russia is placed in the Permian by Lapparent and 

 Geikie, but in the Carboniferous by Tschernyschew. a distinguished Rus- 

 sian geologist. 



Similar difficulties are encountered in various parts of the world by 

 geologists when they attempt to draw the line between the Paleozoic and 

 the Mesozoic systems. Chamberlin and Salisbury (Geology, ii, p. 631) 

 have this to say: 



The Permian system of Europe seems to be more closely allied, strati- 

 graphically, with the Trias than with the Carboniferous, and while the 

 same is true of the western part of America, the opposite is true for the 

 eastern part. 



We have the statement of Geikie (Text-book, p. 1084) that in some 

 regions, as in England, no very satisfactory line of demarcation can al- 

 ways be drawn between Permian and Triassic rocks. 



Nor are geologists free from embarrassments when they endeavor to 

 classify the Mesozoic and the Tertiary formations. The Rhsetic is ar- 

 ranged by Geikie in the Triassic, by Lapparent in the Jurassic. Clark and 

 Bibbins express doubt regarding the position of the two lower divisions of 

 the Potomac formation of the eastern United States. They refer them 

 provisionally to the Jurassic ; the other two divisions are unhesitatingly 

 placed in the Lower Cretaceous. According to Chamberlin and Salisbury, 

 the fossils of the Trinity division of the Comanehean system have raised 

 the question of its reference to the Jurassic. An indefinite number of 

 similar cases could be cited. 



The illustrations presented show that the great divisions of geological 

 record are not even commonly separated by physical breaks, great or 

 small. It would be quite as easy to show that important unconformities 

 occur within the limits of systems of rocks. A few cases only need be 

 cited. The following is quoted from Geikie (Text-book, p. 1007) : 



The Old Red Sandstone of Britain, according to the author's re- 

 searches, consists of two subdivisions, the lower of which passes down 

 conformably into the Upper Silurian deposits, the upper shading Off in 

 the same manner into the base of the Carboniferous system, while they are 

 separated from each other by an unconformability. * * * [In Scot- 

 land] it consists of two well-marked groups of strata, separated from each 

 other by a strong unconformability and a complete break in the succes- 

 sion of organic remains. 



