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



cloudy material, another type of zoning is occasionally seen l both 

 in dolomites from Mango (Fiji), and also on a smaller scale in 

 certain of the Tyrol dolomites. Before a rock is stained, it may 

 appear to contain perfectly-homogeneous, clear, idiomorphic crystals 

 of dolomite. Staining with Lemberg's solution shows, however, 

 that the crystal is composite in character, and is composed of 

 alternate layers of calcite and dolomite (less frequently chalybite 

 forms one of the mineral-layers). The layers are in optical and 

 crystallographic continuity, and the whole appears to form a single 

 crystal. 



Chemical Changes in the Limestones and Dolomites. 



Compared with recent coral-limestones, these Triassic rocks 

 present one or two interesting, if minor, points of difference : — 



1. Only very minute traces of calcium-phosphate are found. 



In coral-limestones *2 per cent, is a fairly- common amount, 

 and occasionally beds of phosphate are met with, as at 

 Christmas Island. The circumstance that the earliest bird 

 so far discovered is of Kimeridge age, is sufficient to account 

 for the absence of beds of limestone altered to phosphate in 

 the area of the Dolomites. 



2. Eecent coral-limestones show no trace of siliciflcation. This 



is remarkable, in view of the fact that among the living 

 organisms present in coral-reefs, siliceous forms sometimes 

 constitute at least 1 per cent, of the whole. The colloid 

 silica of which their skeletons are composed must be peculiarly 

 soluble under the conditions existing in a coral-reef, for no 

 trace of siliceous organisms has ever been found in a recent 

 coral-rock. The same is true of the Tyrol limestones and 

 dolomites, with the significant exception of the quartz- 

 crystals occurring in the coral-section of No. 37. This 

 quartz may represent the colloid silica of siliceous organisms, 

 redeposited in the crystalline state. 



3. In the Tyrol dolomites dolomitization is often more complete 



than in the case of any recent coral-limestone. From 

 Christmas Island 2 and from the Funafuti boring, two dolo- 

 mites which were analysed gave just over 43 per cent, 

 of magnesium-carbonate; but, in general, a condition of 

 stability ensued when the percentage of magnesium-carbonate 

 amounted to about 40. Many of the Tyrol rocks are 

 pure dolomites, containing 45*65 per cent, of magnesium- 

 carbonate, but no rock has been found to exceed the latter 

 amount. 



1 E. W. Skeats, Bull. Mus. Comp. Zool. vol. xlii (1903) pp. 117-18. 



2 Ibid. pp. 96, 102. 



