﻿Vol. 6 1.] DOLOMITES OF SOUTHERN TYROL. 113 



have an area which, after the cessation of volcanic activity in 

 St. Cassian times, consisted probably of shallow-water depressions 

 and lagoons in which the subsequent deposit was laid down. At 

 first, no doubt, the reefs were contaminated by detrital fragments 

 from the volcanic deposits in the neighbourhood, and this is borne 

 out by a chemical examination of the deposits. As subsidence and 

 deposition continued, the volcanic detritus was gradually sealed 

 up, and the subsequently-deposited material became freer from 

 residue, as is seen from an inspection of the results of the analyses 

 from higher horizons. It may or may not be significant that the 

 later-formed limestones have been more completely dolomitized than 

 the older deposits in this area. 



The St. Cassian District. 



The analyses would appear to indicate that the Lavarella mass 

 is free from residue, and is probably of coral-origin. At Sett Sass, 

 south of St. Cassian, the first-formed deposit, like that of the Sella, 

 was mixed with insoluble material, and the origin of the two deposits 

 appears to be the same. The Riehthofen Reef, a lenticular mass in 

 the earthy St. Cassian Limestone, below the dolomite of Sett Sass, 

 appears to have been formed during a quiescent period, in clear 

 water devoid of sediment. 



The Cortina District. 



The faulted dolomite-mass of Col Crepa and Belvedere contains 

 little or no residue, and appears to have been a coral-limestone 

 formed rapidly in clear water. The analysis from the Hexenfels rock 

 near Cortina also indicates a dolomite of great purity. The remain- 

 ing analyses, from scattered localities in the neighbourhood, show 

 generally more residue, and the rocks were possibly deposited in a 

 coastal area supplying detrital material to the deposits. 



The Diirrenstein Area. 



The dolomite in this northern part of the district is much thicker 

 than near Cortina. Rapid deposition under reef-conditions in a 

 subsiding area would appear to have set in at the commencement 

 of the period (an analysis from the base of the Strudelkopf shows 

 no residue), and to have continued until the close, with intermittent 

 deposits into which detrital matter was introduced (Nos. 68 & 69). 



The Drei Zinnen Area. 



The specimens from the Rienz Valley at the base of the Drei 

 Zinnen are dolomites of great purity, in all probability deposited 

 under reef- conditions. 



We have now to consider the composition of the beds below and 

 above the Schlern and St. Cassian Dolomites. 



In areas outside the Schlern, the Langkofl, and the Marmolata, 

 Q. J. G. S. No. 241. i 



