116 



LORD HOWE ISLAND. 



residences of Messrs. Wm. Nichols and T. E. Wilson. Even on the low 

 ground of Moseley's Flat such outcrops are visible, proving ^ that although 

 large areas may be hidden by a deposit of soil, or younger alluvium, the Coral- 

 sand rock is not far below. Furthermore, where the soil is deeper and more 

 plentiful, and from the colour of the latter, it would be difficult to assert 

 that the subjacent rock in situ, would be coral-rock or basalt, pieces of the 

 former are constantly turning up through the operations of the agriculturist. 



Lithologically the 'Coral-sand rock consists of minutely comminuted and 

 completely rounded coral debris, with grains of volcanic substances, such as 

 augite, magnetite, and altered lava, with occasional fragments of echinoderm 

 plates and spines, shells, foraminifera, and particles of sand, bound together 

 by a calcareous paste, consi--ting of a clear crystalline calcite, which 

 does not, however, entirely fill the interstitial spaces, but is sufficient to 

 cement together adjoining grains.* The grains are usually white, but some- 

 times stained yellow, invariably oblong in form, and as a rule, perfectly 

 opaque. So little cohesion is there between the particles, which are of a 

 tolerably uniform size, that it was with the greatest difficulty slides 

 sufiioiently thin could be prepared for the microscope. The disintegrated 

 particles have been very carefully examined by Mr. Whitelegge and the 

 writer, and we believe that a large portion are composed of fragments 

 of nuUipores, and corallines (calcareous algre), with a moderate admixture 

 of true coral debris. Speaking generally the constituents of the Coral- 

 sand rock agree very closely with the component particles of the present 

 beach sand at Lord Howe. One important feature of the latter, is the 

 paucity of foraminifera. In places the Coral-sand rock becomes very loose, 

 and the particles are easily disintegrated. At other times it is very 

 coherently bound together, and becomes a firm solid rock. The fragments 

 appear to owe their rounded aspect not alone to attrition, but probably 

 partly to the solvent action which yields the crystalline calcite for the 

 partially cementing medium. At least 90 per cent, of the component frag- 

 ments appear to be comminuted coral and nuUipores. 



The calcareous condition of this rock is in keeping with that of many 

 other so-called coral " sand-rocks " of comparatively recent origin, but it is 

 not oolitic, and differs from the majority in not having the grains invested by 

 a pellicle or husk of calcite. Thus, at Ascension, and to some extent at 

 St. Helena, in the superficial calcareous deposit there forming, "each 

 rounded particle of shell and volcanic rock can be distinctly seen to be 

 enveloped in a husk of pellucid carbonate of lime."t The same phenomenon 

 i's seen in the recent coral formation at Norfolk Island, J and numerous 

 other instances could be cited. In the large quantity of coral detritus, which 

 acts as its chief constituent the Lord Howe Coral-sand rock resembles the 

 recent limestone of the Bahamas, but essentially differs from that of Bermuda, 

 which, although calcareous, consists of shells and foraminifera. § It is also 

 similar to the blown sands forming the extensive dunes of the South 

 Australian Coast. || Touching the deposits at Ascension and Norfolk 



* The place of tJie calcite as a ccnipntinc; medium is occasionally taken, althoiigli not 

 often, by a dark volcanic paste of a similar colour to the included volcanic grains. The 

 structure of this rock has already been described by Mr. T. Davies and the -writer, in a 

 paper bv Sir R. Owen, " Description of the Fossil Eemains of two species of a Megalaniaii 

 aenus, Meiolania, from Lord Howe Island." FUl. Trans., 1887, cixxvn, p. 497. 



t Darwin, Geol. Obs. Vole. Islands, 1844, p. 49. 



X J. E. Carne, Ann. Eeport, Dcpt, Mines, N.S. Wales, for 1883 [1886], p. 145; T. W.E. 

 David, Hid, p. 147. 



§ Nelson, Quart. Jonrn. Gcol. Soc, 1853, IX, p. 207. 



I| Teu.-Woods, Geol. Obs. S. Australia, p. 182., 



