REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 73 



the calcite cementing them, by their internal structure, semi-opacity, and greyish tint. 

 The internal structure of the fragments is generally well preserved ; sometimes they 

 present a spathic cleavage, and at others the calcium carbonate composing them is very 

 fibrous. Yet, as examination in convergent light shows, it is impossible to refer these 

 fibrous sections to aragonite. With the condenser, one arm of the cross of monaxial 

 crystals may be seen. The rolled fragments of inorganic origin cemented together with 

 the shell sand are the debris of volcanic minerals or rocks. The latter are most 

 frequently represented by rounded spangles of plagioclastic felspar, often by grains of 

 olivine, but augite is rather rare. The lapilli, or rolled fragments of rocks, belong 

 generally to the family of basalts. They are scoriaceous, often vitreous, and trans- 

 formed into palagonite with vesicles lined with zeolites. Boiled fragments of traehytic 

 rocks rarely occur in this limestone ; this may be accounted for by the fact that basalt 

 chiefly occurs on this side of Ascension. The rocks and minerals enclosed in the calcite 

 are all somewhat profoundly altered. The substance cementing these heterogenous frag- 

 ments is always calcium carbonate, perfectly transparent and fibrous ; this distinguishes 

 it at the first glance from the included shell-particles. The fibres are so fine that it 

 is impossible by optical means to determine whether they are calcite or aragonite ; the 

 polarisation colours and the irisation are the same as for calcite. The calcareous coat 

 which envelops each of the rolled grains is sometimes fibro-radiated, the fibres spread- 

 ing from one grain to the sides of the zone surrounding the contiguous fragments. 

 The calcareous matter sometimes does not fill all the interstices, and the resulting 

 little geodes, sometimes of triangular form, bristle with a fine lacework of rod-shaped 

 crystals of calcium carbonate. 



In conclusion, something must be said about a shining coating of calcium phos- 

 phate which clothes some of the rocks of Ascension. In his description of the rocks of 

 St. Paul, Darwin drew attention to an enamel coating which covered the cliffs of that 

 islet. We have described and analysed the material which Darwin found at St. Paul's 

 Rocks, and compared it with the substance coating the rocks of Ascension. Darwin, 

 describing this glossy incrustation, says : " Extensive portions of these rocks are 

 coated by a layer of a glossy polished substance, with a pearly lustre and of a 

 greyish - white colour ; it follows all the inequalities of the surface, to which it 

 is firmly attached. When examined with a lens, it is found to consist of numerous 

 exceedingly thin layers, their aggregate thickness being about the tenth of an inch. It 

 is considerably harder than calcareous spar, but can be scratched with a knife ; under 

 the blowpipe it scales off, decrepitates, slightly blackens, emits a fetid odour, and 

 becomes strongly alkaline : it does not effervesce in acids. I presume this substance 

 has been deposited by water, draining from the birds' dung with which the rocks are 

 covered. At Ascension, near a cavity in the rocks, which was filled with a laminated 

 mass of infiltrated birds' dung, I found some irregularly-formed stalactitical masses of 



(PHYS. CHEM. CHALL. EXP. — PART VII. — 1889.) 10 



