CORRELATION AND UNCONFORMITY. 79 



qualified by the geographic phase during which the organisms lived. In 

 the world-wide Carboniferous seas marine organisms attained cosmopolitan 

 distribution; and correlation between Europe, Asia, and America rests 

 upon a firm paleontologic basis; but when those seas, so far as their epi- 

 continental parts were concerned, became transformed into bays, straits, 

 carribbeans, and mediterraneans, organisms met with great variety of expe- 

 rience. Where favored they persisted or slowly evolved; where adversely 

 conditioned they died out or varied, and cosmopolitan relationships gave 

 place to provincial diversity. That they did so sooner here and later 

 there, and that the old sometimes returned to a habitat from which it 

 had been driven during a temporary unfavorable condition, is the record of 

 stratigraphy and paleontology.* It follows that correlations on a paleon- 

 tologic basis are less reliable during the transition from Paleozoic to Meso- 

 zoic than during those periods of more general conditions which preceded 

 and followed; and that even though Permian and Triassic faunas were 

 more abundant and better known than they are in Asia, we should still 

 have difficulty in establishing a plane with reference to which, in central 

 Asia, China, Australia, and India, we could say: that which is beneath is 

 Paleozoic and that which is above is Mesozoic. There is no such general 

 plane. 



Unconformity is a condition common to a transition stage, regarding 

 which a word is necessary in explanation of the diversities which Chinese 

 sections present. The term covers a great range of phenomena indicating 

 discontinuity of deposition, from that which may result from nondeposi- 

 tion, marine scour, or subaerial erosion, without obvious disturbance, to the 

 most striking differences of structure. We are apt to think of a submerged 

 surface as one subject to sedimentation, or vice versa of a surface which 4 

 has not received sediment as one which must have been above water; but 

 the inference is not valid in view of the fact that marine currents, when 

 confined in straits or shallows to such an extent that the water next the 

 bottom moves, are capable of carrying sediment past a district, or even of 

 scouring the bottom. Considering the work of corrasion done by rivers 

 whose depth and bottom pressure are relatively insignificant, the capacity 

 of marine currents to scour can not be questioned, provided the waters are 

 equipped with abrading materials in the form of silt, fine sand, or siliceous 

 spicules.f Nondeposition during an indefinite time or corrasion of an 

 undisturbed stratum may result. It is thus that we may explain the 

 contrast of strata where the Carboniferous limestone is concordantly over- 

 lain by red terrigenous deposits, as in eastern Ssi'-ch'uan. The limestone 



* Williams: Devonian Section of Ithaca, New York, Journal of Geology, vol. xv, p. m, October, 1906. 

 t Agassiz: Three Cruises of the Blake, vol. i, pp. 136-139. 



