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



be recognized, each being characterized by some mineralogical 

 peculiarity. 



1. Many of the more recent limestones, while consisting largely 



of calcite, contain also organisms, the skeletons of which 

 consist of aragonite; and not infrequently secondary aragonite 

 has been deposited in crystallographic continuity with the 

 aragonite of the organism. 



2. Gradual alteration and recrystallization of the first type of 



limestone leads to the production of a rock in which organic 

 remains and matrix alike consist entirely of calcite. 



3. The replacement of calcium-carbonate by magnesium-car- 



bonate in limestones either of Type 1 or Type 2 leads to the 

 formation of a third type, characterized by the presence 

 of crystals of dolomite ; in some cases, the replacement 

 proceeds so far that the limestone is entirely converted into 

 dolomite. 



(1) Examination of many sections of rocks belonging to the 

 horizon of the Schlern Dolomite shows that the first type of lime- 

 stone, characterized by the presence of aragonite, is now wanting. 

 The St. Cassian Limestone of Sett Sass, however, provides a very 

 interesting example of the aragonitic type of limestone. A section 

 of a coral, when examined under a high power, shows a remarkable 

 preservation of the original aragonite-fibres of the coral and of the 

 secondary aragonite formed in crystallographic continuity with 

 them. With this single exception, none of the rocks examined 

 contained any crystals of aragonite. 



(2) The Marmolata massif affords good examples of limestones 

 consisting entirely of the mineral calcite. The rocks have been 

 much recrystallized as a result of earth-movements, so that lamellar 

 twinning is very common in the calcite-crystals, which are generally 

 allotriomorphic. Many of the St. Cassian Limestones also consist 

 entirely of calcite and calcite-organisms. 



(3) Among the rocks in which the mineral dolomite occurs we 

 have : — 



(a) Dolomitic limestones, and (/3) Dolomites proper. 



In rocks belonging to the group a magnesium-carbonate gene- 

 rally forms from 10 to 30 per cent, of the mass, so that a larger 

 or smaller amount of calcite is always present. In these lime- 

 stones, the dolomite-crystals are generally markedly idiomorphic. 

 One of the Marmolata rocks (No. 32) shows fine idiomorphic 

 dolomite-crystals, surrounded by a matrix, of calcite (PI. X, 

 fig. 2). The same feature is seen on a minute scale in No. 77, 

 one of the St. Cassian oolitic limestones, in which tiny rhombo- 

 hedra of dolomite occur, and are restricted to the matrix of the 

 rock (PI. XI, fig. 2). No. 34, one of the Marmolata rocks, is a 

 limestone containing broken fragments of dolomite- crystals which 

 have no definite extinction, and are possibly of detrital origin. 

 No. 37, a dolomitic limestone from the Giau Pass, contains a 

 longitudinal section of a coral with dolomite-crystals deposited in a 



