MINERALOGICAL CHANGES OBSERVED IN CORES OF FUNAFUTI BORINGS. 405 



to the complete, or almost complete exclusion of recognisable calcite. At three 

 horizons, however, calcite is present in considerable proportions, causing the percentage 

 of magnesium carbonate to fall below this maximum, and rendering it possible to 

 divide the rocks into an alternating series of dolomites of fairly constant composition 

 and dolomitic limestones''^ of variable composition. 



Fig. 39.— Main Boring. Core 313. Depth 637 feet. 

 X 200. 



Section of the core immediately overlying that in 

 which dolomite crystals first make their appear- 

 ance. The rock is very porous. The cavities 

 are lined with well-formed scalenohedral crys- 

 tals of secondary calcite, which are often de- 

 posited in crystallographic continuity with the 

 structural elements of the organisms. In the 

 more solid parts of the mass the secondary 

 calcite is granular in character. 



Fig. 40.— Main Boring. Core 314. Depth 638 feet. 

 X 250. 



Section of the highest core in which recognisable 

 dolomite occurs. The organisms and their in- 

 vestment of acute crystals are of calcite. The 

 rest of the mass consists of crystalline dolomite. 



- (a) The Replacement of Calcite and Aragonite by Dolomite, with the Formation 



of Casts. 



The first core in which crystallised dolomite is seen comes from the depth of 

 638 feet, and contains 20 "44 per cent, of magnesium carbonate. This, if assumed to 

 be all combined with calcium carbonate to form the double carbonate of calcium and 

 magnesium, indicates that nearly one-half of the rock here is composed of dolomite. 

 The core above, from 637 feet, shows no dolomite, and contains only 2 '44 per cent, of 



* The term dolomitic limestone is here employed to denote a rock consisting of both calcite and 

 dolomite, as distinguished from the dolomites which consist of the mineral dolomite only, though not 

 necessarily of pure dolomite. 



