﻿120 PROF. E. W. SKEATS ON THE [Feb. I905, 



irregular spaces and cracks filled with clearer secondary dolomite. 

 Scattered through the rock are some crystals of pyrite, altered to 

 oxide of iron. 



Wo. 108. Base of Sett Sass, facing the Richthofen Reef. 



Part of the slide consists of allotriomorphic, part of idiomorphic 

 crystals of dolomite, with regularly -arranged dark centres, set in a 

 matrix of a muddy-looking material. Cracks have been filled with 

 clear allotriomorphic secondary crystals, while a little pyrite with 

 an aureole of iron-oxide is also noticeable. 



The Richthofen Reef. 



No. 110. Prom the outer face of the Richthofen Reef. 



The rock has a cloudy granular matrix, in which are seen large 

 cloudy rhombohedra of dolomite with light borders. Irregular 

 cavities are lined with large, clear, secondary dolomite-crystals. 



St. Cassian Limestone from Sett Sass. 



No. 109. St. Cassian Limestone with corals (?), between Sett Sass 

 and the Richthofen Reef. (See PI. XII, fig. 2 & PI. XIII, fig. 1.) 



The slide consists mainly of a transverse section of a coral; and a 

 longitudinal section of one of the calcareous algse, probably Sphcero- 

 coclium, is also present. The coral is embedded in a matrix of 

 calcareous mud, part of which still remains, but most of it has 

 clarified by recrystallization as a clear mosaic of calcite. This 

 muddy matrix has served to prevent the degradation of the coral „ 

 the structure of which, for the most part, is wonderfully preserved. 

 The original aragonite-fibres of the stereoplasm, the brown colour 

 of the septa, the centres of calcification, and even the secondary 

 aragonite, subsequently deposited in optical continuity with the 

 coral-fibres, remain unaltered. The appearance of the coral in 

 section is such, that it is difficult to believe that one is dealing, 

 not with a coral from an unaltered modern reef, but with one of 

 Triassic age. 



The Cortina District. 

 Col Crepa. 



No. 49. Top of the western part of Col Crepa. 



"While the section shows a certain amount of a fine, granular, 

 dark matrix, most of the slide is comprised of clear allotriomorphic 

 dolomite-crystals, in which no organisms can be detected. 



No. 51. Slickensided and brecciated rock from a fault-plane, 



few yards below No. 49. 



Large and small, composite, angular fragments of clear dolomite 

 are set in a fine dolomitic matrix. The larger fragments consist 

 of several allotriomorphic crystals, and some of these show evidence 

 of lamellar twinning in polarized light, a very unusual occurrence 

 in dolomite. 



