REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 71 



tion. The dust of the earthy variety, and the slices of the concretionary, are filled 

 with elongated colourless forms, more or less rounded, and slightly curved ; these are 

 undoubtedly organic and siliceous ; they are the debris of the organisms which Ehren- 

 berg discovered and determined. These particles are enclosed in a pale yellowish 

 isotropic matrix without definite outline. When this opaline ground-mass is more 

 coherent, one sees that the rods and colourless organic forms appear as partly dis- 

 solved ; the ground-mass is more homogeneous, and the interstices are lined with 

 microscopic grains of quartz. Splinters of glass, lapilli, or minerals of volcanic origin 

 are rarely seen. 



Ehrenberg's explanation does not seem to apply here ; there is nothing to indicate 

 an eruptive origin for the siliceous earth and its nodules. It seems more reasonable 

 and more probable to admit that the cavity containing the deposit in question was 

 formerly a crater-lake, in which the remains of fresh-water organisms accumulated ; 

 part of the constituent silica was dissolved, perhaps under the influence of thermal 

 springs, and cemented the particles which in aggregating took in some cases the form 

 of nodules. 



Calcareous Eocks Forming on the Coasts. 



Darwin describes calcareous rocks in process of formation at several points on the 

 coast of the island. 1 The shore is covered with immense numbers of minute rounded 

 particles of shells and coral, white, yellow, and red in colour, mixed with rounded 

 volcanic minerals and splinters. At a depth of some feet the particles are cemented, 

 and form a compact rock, the softest kind of which is used for building, while some 

 varieties are too hard for this purpose. One of these calcareous masses was observed 

 divided into horizontal layers half an inch thick ; it gave a ringing sound like flint 

 under the hammer. The people of the island believe that one year suffices to cement 

 the calcareous sand into stone. The sand is united by a calcareous cement, and one 

 can always observe, even in the most compact varieties, a zone of crystalline calcite 

 around every fragment of shell and each volcanic grain. Lyell 2 states that turtles' 

 eggs deposited in this calcareous and volcanic sand are sometimes subjected to the 

 same process, and are found enclosed in the mass. He has figured some eggs contain- 

 ing the bones of young turtles that were included in this way in these recent calcareous 

 rocks. Darwin treated a specimen of the rock of specific gravity 2'63 with acid, 

 and found that it dissolved entirely with the exception of a little flocculent organic 

 matter. 



A great accumulation of calcareous particles takes place annually on the shore near 



1 Darwin, Geol. Obs., pp. 49, 50. 



2 Lyell, Principles of Geology, Book III. chap, xvii., as cited by Darwin ; in Lyell's edition of 1872, see vol. ii. 

 chap, xlyiii p. 581. 



