118 W. B. Clark — Ammonite from the Alpine Rho&tic beds. 



Art. X. — A new Ammonite which throws additional light 

 upon the geological position of the Alpine Hhcetie ; by 

 William B. Clark, 



In a paper* upon the geology of a part of the northern 

 Tyrol, published in Munich in February, 1887, I described 

 among several new forms a species of ammonite of the genus 

 Arcestes, which is of some considerable importance, as pointing 

 to the probable position of the Rhsetic beds. 



In the region above mentioned this much debated formation 

 consists of the three typical divisions of Haupt Dolomit, 

 Kossener Schichten and Dachstein JTalk, the lower or Haupt 

 Dolomit being plainly subdivided into a zone of dolomite of 

 somewhat over 1,000 feet in thickness, overlaid by a thinner 

 zone of limestone, the so-called Platten Kalk of Gumbel. 

 The lower zone is probably unfossiliferous, with the exception 

 of some interstratified beds of asphalt which contain ganoid 

 scales ; while the upper, although containing numerous ill-de- 

 fined gasteropods, doubtless of the genus Hissoa, affords no 

 distinctive forms that would of themselves demand a close 

 union either with the underlying Trias or overlying Jura. The 

 Kossener Schichten, in contradistinction to the lower division, 

 are very fossiliferous and present the chief ground of discus- 

 sion. The rock is a dark limestone, often of a marly, schistose 

 character, with frequent interstratified beds of marl, that grades 

 down insensibly into the Platten Kalk. The fossils show nu- 

 merous affinities both with Triassic and Jurassic forms, but for 

 the most part appear to be of the former character. They are 

 largely corals, brachiopods and lamellibranchs, and the exam- 

 ple here cited is the first case of a well-defined ammonite. 

 The genus Choristoceras, so commonly encountered, is a degen- 

 erate form placed with the Ceratitidce, and has small value in 

 this connection. The third division, the Dachstein Kalk, is of 

 white limestone, and contains almost exclusively lithodendron- 

 like corals, though the large Megalodon triqueter, the charac- 

 teristic bivalve of this zone, is frequently encountered. It is 

 without doubt of coral-reef origin, as many physical facts 

 along its contact with the Lias give proof. 



Facts seem to indicate that these different divisions, certainly 

 so far as the Kossener Schichten and Dachstein Kalk are con- 

 cerned, are only facies, and may under suitable conditions be 

 interchangeable. 



* Ueber die geologischen Verhaltnisse der Gegend nordwestlich von Aachensee 

 mit besonderer Beriicksichtigung der Bivalven und G-asteropoden des untereu 

 Lias. 8°. 45s., 2 taf., 1 Karte. Munchen, 1887. (Inaugural Dissertation). 



