REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 69 



singular character to this eminence. The outer ring is large, distinguished by its white 

 colour and its resemblance to a racecourse, — hence the name of Devil's Eiding School. 

 According to Darwin these beds of ashes must have covered the whole region formerly, 

 but they have been dispersed by wind — those which had fallen into the hollow on 

 the summit were sheltered, and became to a certain extent cemented and consolidated 

 by rain. One of the beds has a rosy colour, and is formed essentially of small frag- 

 ments of pumice. It contains numerous concretions, which are spherical and vary 

 from half an inch to three inches in diameter ; sometimes they are cylindrical, like 

 the concretions of pyrites in the chalk. These concretions are formed of six or 

 eight clearly-defined concentric layers, separated by colourless zones, and surrounding 

 a nucleus which appears to be homogeneous. The central part is often traversed by 

 fissures bike those of septaria ; these are bordered by black veinules, which sometimes 

 assume a metallic aspect, or by white patches. Amongst the largest concretions, some 

 were found which simply formed a spherical shell full of incoherent volcanic ashes. 

 These concretions contain only a small proportion of calcium carbonate. Before the 

 blow-pipe a fragment crepitates, whitens, fuses into a frothy enamel, but does not 

 become caustic. The mass enclosing the nodules contains no trace of calcium carbonate. 

 Darwin adds that he never met with a description of similar nodules, and what 

 rendered them the more remarkable, in his estimation, was their hardness and com- 

 pactness, which must have been acquired under the influence of atmospheric water 

 alone. 



So far, with regard to these concretions, we have only cited Darwin, whose descrip- 

 tion corresponds very exactly with the facts he observed. At the time of publishing 

 his book on Volcanic Islands, he considered these spheroidal concretions, and the 

 material with which they were associated, as exclusively made up of incoherent 

 volcanic products. After his voyage he submitted a specimen of the concretions to 

 Ehrenberg. Microscopic examination showed that it did not present the characters 

 of ordinary volcanic ashes, but that the rock was only an accumulation of particles 

 of organic origin. According to Ehrenberg, these particles are not very much modified, 

 although they no longer contain any compounds of carbon. He attributed the 

 elimination of these bodies to the action of heat. He did not admit that these 

 organisms periodically accumulated in the hollow, as it would be necessary to suppose 

 if they lived where their remains were discovered. The whole mass was apparently 

 formed of organic debris, and Ehrenberg observed 30 species of siliceous organisms 

 in the deposit. He even considered the more or less amorphous matter which is 

 associated with the particles as being exclusively composed of this siliceous debris 

 in a state of dust. These organisms all belong to fresh-water forms, the greater 

 number of small siliceous particles being derived from grasses. It is very remark- 

 able that no marine forms have been discovered on this island. In concluding his 



