MINER ALOGICAL CHANGES OBSERVED IN CORES OP FUNAFUTI BORINGS. 303 



bv means of which aragonite, if present in a section, is stained, while calcite and 

 dolomite remain unstained. The second is that of Lemberg,'"' by which calcite and 

 aragonite are stained, while dolomite remains unstained. The former method serves 

 for the differentiation of aragonite from calcite and dolomite ; the latter for that of- 

 dolomite from calcite and aragonite (see Plate F). 



The value of these two tests is increased by the fact that they provide a simple and 

 trustworthy means of determining the mineralogical nature, not only of the well-crystal- 

 lised inorganic portions of the rocks, but also of the original calcareous organisms. 

 Concerning the organisms of the Funafuti reef — as exemplified in the rocks of the 

 boring — the tests have given the following results. Those composed of aragonite, 

 are the calcareous alga Halimeda, the Madreporarian corals, the alcyonarian genus 

 Heliopora, the hydrocorallines, the majority of the mollusca, and the minute spicules 

 of tunicates. The rest are of calcite, and comprise the calcareous algse Litho- 

 thamnion and Lithoj)hyUum, the foraminifera, the polyzoa, the echinodermata, and the 

 spicules of alcyonaria. These results, so far as they go, are in agreement with those 

 obtained in the first instance by Dr. IT. C SoRBY,t and by numerous investigators 

 later. 



The calcite and aragonite of which the various organisms are composed may be 

 spoken of as primary calcite and aragonite ; that which makes its appearances in the 

 rocks after their accumulation will be referred to as secondary calcite and aragonite. 

 No such distinction is necessary in the case of the dolomite, since it is always of 

 secondary origin. 



The distribution of these three minerals in the boring is noteworthy. Aragonite is 

 confined to the upper cores, and dolomite to the lower, while calcite, which is the sole 

 constituent of the middle cores, is also found above and below, in association with 

 aragonite on the one hand and dolomite on the other. Dolomite is not encountered 

 in a clearly individualised form, until a depth of between 637 and 638 feet has been 

 reached ; below this it continues, in varying amounts, to the bottom of the boring. 

 It may be merely a coincidence, or it may be a fact of special significance, that in none 

 of the cores has aragonite been found in association with dolomite. 



2. The Mineralogical Characters of the Rocks occurring between 

 THE Surface and the Depth of 637 Feet. 



The rocks included within these limits are limestones, which contain, as a rule, 

 only a small proportion of magnesium carbonate ; but within the first 50 feet they 

 carry a higher proportion than usual of this constituent, which attains a maximum 

 of 16 per cent, at the depth of 25 feet. From that point, both upwards and down- 

 wards, the percentage of magnesium carbonate decreases, till it reaches its normal 



* ' Zeitsch. Deutsch. Geol. Gesellsch.,' 1888, vol. 40, p. 357. 



t Presidential Address, ' Quart. Journ. Geol. Soc.,' 1879, vol, 35, Proc. pp. 58 — 66, 



3 E 



