282 
The Artinskian of Russia is placed in the Permian by Lapparent and 
Geikie, but in the Carbonifercus 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 Rheetice is ar- 
ranged by Geikie in the Triassic, by Lapparent in the Jurassic. Clark and 
3ibbins 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 Comanchean 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. 
