I.—Coral in the “Dolomites” of South Tyrol

Abstract

The Raibl period was the natural sequel of the variable and unequal movements which prevailed over Alpine areasinPermian and pre-Raibl Triassic time. Many basins formerly open were then enclosed; rauchwackes and beds of dolomite and gypsum were interbedded with fossiliferous deposits. Whereas,insome places, the dolomitic nature of the deposit is confined to special horizons, in the South Tyrol “Dolomites” it may almost be said to reign throughout. This makes it all but impossible to say when Schlern dolomite ends and Raibl beds begin. in the present incomplete state of our knowledge with regard to the heteropism of the Raibl series throughout the whole Alps, I have judged it best to begin the Raibl horizon at any particular place with the first appearance of a distinctly Raibl fauna, even although that fauna may not have been proved to correspond to the acknowledged lowest fauna of Raibl age in distant parts of the Alps.To return for a moment to the succession of Schlern dolomite upon the Cassian beds of Enneberg, I found that, where Schlern dolomite rests on Cipit limestones, it has at its base a conglomeratic appearance, as if Cipit blocks had been imbedded in a beautifully fine white or reddish dolomitic mud, instead of the dingy brown and black tufaceous sediments. This is the case in several places, e.g. upon Pordoi and Sella Jochs, where there is no evidence of unconformity. Again, where the dolomite succeeds the thin-bedded marls and limestones of Cassian age, it does so conformably; but one and the same bed is at some parts calcareous and fossiliferous, at other parts dolomitic and unfossiliferous. Seeing that this holds good at various horizons in Lower as well as Middle Trias over the whole area of South Tyrol, we need f in d nothing remarkable in it from the point of view of the stratigraphical succession. Indeed, I have only mentioned these observations as an indication of the particular mode of transition from conditions of deposition favourable for the Cassian fauna to those in which the Raibl fauna was enabled to make an occasional appearance in the South Tyrol dolomites. At a very little distance above the base of Schlern dolomite all signs of Coral life disappear, and the deposit looks a homogeneous rock, although always retaining local variation in the degree of its dolomitism. At this stage the rock often shows typical Oolite structure. As regards the presence or want of stratification, it has as little to do with the question of the Coral Keef origin of the dolomite as the amount of magnesic salts in the rock—stratification is present and absent in one and the same “Keef.”