GEOLOGY. 



121 



Wlien seen on the shore it is of a dark ochreous colour, decidedly argillaceous 

 and stiff, containing bird's bones (chiefly Puffinus), and piimice. 



Mr. Wilkinson gives its composition as 25-1 per cent, oi phosphoric acid, 

 equal to (Oa, V^ Ob) Tribasic Phosphate of Lime. 



It will be remembered that the Coral-sand rock contained phosphoric acid up 

 to fourteen per cent., and it is probable that this important and valuable con- 

 stituent is to some, although small extent, derived from the disintegration of 

 the coral deposit, which is one of its constituents as well as bird remains and 



exuvise. . , . , . , , • ,1 



The formation of this phosphatic deposit, which is much too impure to be 

 of any commercial value, is, to some extent, explained by the present mutton- 

 bird roolvery at Clear Place Point. Here, where several thousand birds are 

 at work, excavating their burrows, sometimes in the decomposed Coral-sand 

 rock, sometimes in the volcanic soil, intermingled _ with their bones and 

 droppino-s, a thin but semi-phosphatic accumulation is going on. 



Loam" Soil, and Alluvium— In many places surrounding the low, scarp- 

 like outcrops of the Coral-sand rock, but more usually covering the latter, is a 

 deposit of loam, forming the soil of the island. It is described by Mr. 

 Charles Moore as of a rich character, being of a dark, unctuous, loamy- 

 nature, largely impregnated with humus.* It, however, varies in composi- 

 tion, according to whether derived wholly from the decomposition ot the 

 basalt, or that only in part, combined with the decomposed Coral-sand rock. 

 Decayed vegetable matter enters largely into its composition also. I he 

 analyses of three different samples are given by Mr. H. T. Wilkinson as yield- 

 ing phosphoric acid equal to tribasic-phosphate of lime to the extent ot 20-1 

 per cent., 91 per cent., and 5-6 per cent, respectively, but neither of these is 

 sufficiently rich for commercial purposes as a manure. By dissolving por- 

 tions in water, when of a decidedly chocolate colour, a residue of volcanic 

 products is left. As the basaltic rook is receded from, the soil becomes 

 more and more sandy-calcareous, until at last the loamy nature is lost, and 

 it assumes the aspect of a calcareous, sandy soil. , • i -, ^ 



Eich patches of this deposit afford the best garden sites on the island, and 

 it is during gardening operations that the remains of Meiolama are usually 

 found, lying loose, having been detached from the nearest rock during 

 its denudation. It is in this way that the irregular holes and spaces so fre- 

 quently met with in blocks of detached Coral-sand rock become filled as 



previously described. ii, 42 t-i • 



I am quite unable to give any precise information as to the depth ot this 

 loam, although in places'there must be a considerable thickness. It supports 

 a most luxuriant vegetation. 



The Old Settlement and Deep Creeks, at their mouths traverse small 

 alluvial flats, formed by the detrital matter brought down by their action, 

 Such accumulations must be regarded as quite distinct from the loam deposit, 

 which is the result of atmospheric action on the rocks immediately under- 

 lying, and vegetable decay, and, although both may have been formed more 

 or less concurrently, I have no data on which to base an opmon as to the 

 thickness of these alluvial strata, but it cannot be very great. As m the 

 case of the loam, the flats form excellent agricultural areas. 



Under the term "marine alluvium" can only be included the sandy 

 beaches of the Lagoon, Ned's Beach, Middle Beach, and BlenkinthorpeBay 

 which have already been referred to. 



^olian Deposits.— Along the shore frontage of the Old Settlement, the 

 Lagoon south of Thompson's Point, and Ned's Beach are low sand hummocks, 



* Hill's Lord ITov-e Island, loc. cit., p. 17. 



