4/8 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



Whereas Leopold von Buch had explained these masses 

 as dolomitised limestone, chemically altered by the agency 

 of magnesia vapours and volcanic discharges, Richthofen 

 made the suggestion that not only the dolomitic masses, 

 but also a part of the immense thicknesses of pure pelagic 

 Triassic limestone in the southern Alps, had been constructed 

 by reef-building coral polyps during periods of slow subsidence 

 of the sea-floor. Richthofen pointed out how the irregular 

 constitution of a sea-floor occupied by coral reefs would afford 

 an explanation for many of the peculiar tectonic appearances 

 and facies developments that are otherwise very difficult of 

 comprehension. Richthofen's sub-division of the Trias in 

 South Tyrol has been little altered. Stur, in 1868, showed 

 that the Heiligkreuz strata were parallel with the upper part of 

 the Rai-bl strata ; and as the position of the Kossen strata 

 became fixed, both these and the Dachstein limestone, so often 

 intimately associated with the Kossen strata, were transferred 

 from Lias to Upper Trias. 



Until the year 1856 there was no known extra- Alpine 

 equivalent for any of the zones of fossiliferous Triassic deposits 

 above the Muschelkalk. Athough almost one thousand 

 species of marine fossils had been described' from St. Cassian, 

 Raibl, Esino, and Hallstatt strata, there was not a single species 

 amongst them which could with security be shown to occur in 

 extra-Alpine deposits. The only basis of comparison between 

 Alpine and extra-Alpine Trias had been afforded by the few 

 fossil species common to Alpine and extra-Alpine Muschelkalk. 

 The highest interest, therefore, attached to the publication of 

 a memoir by Oppel and Suess " On the supposed equiva- 

 lents of the Kossen strata'"' (Sitz. ber. Akad. Wien, 1866), 

 wherein Avicula contorta and other Molluscan species in the 

 Kossen beds were identified with species in certain passage- 

 beds between the Triassic and Liassic strata in Swabia. 



There could be no question regarding the stratigraphical 

 position of the Avicula contorta strata in Swabia since they 

 reposed conformably upon the upper red Keuper marls, and 

 were conformably succeeded by the lowest Lias with Am- 

 monites planorbis. Hence the determination of this definite 

 palaeontological zone in the Alps fixed the upper horizon of 

 Alpine Trias, and gave a clue to the solution of the relations 

 between the Trias and the Lias in the Alps. While strati- 

 graphers were well satisfied with this new vantage-ground for 



