﻿Vol. 6 1.] DOLOMITES OF SOUTHERN TYROL. 119 



dolomite-crystals showing dirt-zones. The remaining spaces are 

 sometimes filled with a finer-grained mixture of calcite and small 

 dolomite-rhombohedra, some of which show external zones of calcite. 



No. 81. A very similar rock to No. 82. Meandrine organisms 

 are indicated by the arrangement of dark-centred dolomite-crystals. 

 Cavities in the rock have been subsequently filled with clear, coarsely- 

 crystalline dolomite. 



St. Cassian Limestones from St. Cassian. 



No. 77. Above St. Cassian and below Lavarella. (PL XI, fig. 2.) 

 A well-preserved oolitic limestone. The oolitic grains are well 

 formed, they show both concentric and radiating structure, and 

 their centres in some cases consist of shell-fragments. For the 

 most part, they are not in contact one with the other, but are 

 separated by a cement of calcite in which are embedded numerous 

 very minute rhombohedra of dolomite and chalybite (about grVo i ncn 

 in length). The dolomite and chalybite are practically restricted to 

 the matrix of the rock, while later-formed cracks have been filled 

 exclusively with calcite. 



No. 76. A few feet below No. 77. 



Irregular fragments of partly-disintegrated organisms l (in- 

 cluding Splicer ocodium Bornemanni and other calcareous algae) and 

 pieces of limestone are embedded in a matrix of crystalline calcite. 

 Minute crystals of dolomite occur scattered through the matrix, 

 and some of them have outer zones of calcite. Cubes of pyrite, 

 marginally altered to limonite, are abundant, and cracks traversing 

 the slide are filled exclusively with calcite. 



Sett Sass. (Schlern Dolomite.) 



No. 112. Block fallen from the summit. (See PL XI V, fig. 1.) 

 The section consists of rounded bodies united by a granular 

 dolomitic cement. The rounded bodies are most probably sections 

 of organisms, possibly calcareous algse (Gyroporella ?), which have 

 been completely dolomitized. Whereas the dolomite in the matrix is 

 granular, that within the organisms consists of distinct rhombohedra 

 with cloudy centres. It is noteworthy that dolomite formed 

 by the alteration of a matrix of calcite-crystals is 

 usually clear, while that formed by the alteration of 

 organisms is almost invariably cloudy. The boundaries 

 of the organisms are defined in this section by dolomite-crystals 

 with very dark centres. Empty spaces occur within some of the 

 organisms, and these cavities are lined with clearer secondary 

 dolomite. 



No. 107. Immediately below the junction with the Raibl Beds, 

 northern end of Sett Sass. 



The section consists mainly of cloudy dolomite, in which occur 



1 Kindly determined for me by Dr. G-. J. Hinde, F.E.S. 



