370 PROFESSOR J. W JUDD. 



description of the geology of Funafuti it will be seen that considerable quantities oi 

 pumice are found washed up on the beaches. As is pointed out in the account of the 

 Geology of the Island (pp. 76-77), the Hurricane Bank contains a thin but very 

 persistent band of pumice, formed of materials which were floated across the Pacific 

 after the eruption at Blanche Bay in New Britain in 1878. Dr. Hinde informs me 

 that in no instance during his detailed examination of the cores did he detect any 

 trace of pumice in them ; a particle which he found in loose material may have 

 fallen in from above. The absence of silicates and other insoluble matter in the 

 cores, as shown by their chemical analysis, proves that fragments of pumice and 

 similar volcanic materials are very rarely enclosed in the coral-rock. In all probability 

 such materials, though they may be stranded on the beaches for a time, are washed 

 away again before becoming embedded in the limestone rock. Like all sub-iierial 

 accumulations, they can only be preserved in submarine deposits by a combination of 

 accidents. 



It will be seen from the table given on the preceding page, that the quantities of 

 insoluble inorganic matter in the cores of Funafuti, show no tendency to increase in 

 going downwards. Near the bottom of the core, at a depth of between 1075 and 

 1087 feet, an insoluble residue of only "004 per cent, was obtained, this being about 

 the usual average amount found from top to bottom of the boring, except in one 

 or two special cases. It is fair to conclude, therefore, that the boring, at its greatest 

 depth, exhibited no indication of the proximity of volcanic rock or any materials 

 containing silicates. The atoll is so large that in the centre of the lagoon non- 

 calcareous material may exist enveloi)ed and ])uried in coral-reef rock, and such 

 non-calc<\re()us material may be indicated more or less obscurely by the results of the 

 magnetic survey (see p. 3G) ; but so far as the study of the rocks obtained from 

 the boring affords any means of judgment, there is no more indication of the proximity 

 of anv foreign rocks at the depth of 1114 feet than there was at the surface. 



The analysis of the deep-sea deposits brought home by the " Challenger " showed 

 that in all cases the calcareous deposits from the ocean floor contain a proportion of 

 insoluble matter, which generally varies between at least 1 and 3 per cent. ; Sir John 

 Murray assures me that, as the result of his wide experience, it may be accepted 

 as a proved fact that such a proportion of insoluble matter is characteristic of all deep- 

 sea calcareous deposits. 



Hence, as coral-reef rocks have now been shown I^j be distinguished, except when 

 formed in proximity to volcanic masses, hy the almost complete absence of insoluble 

 inorganic matter, the geologist obtains a criterion by which he may probably be able 

 to distinguish limestones which have been formed on and around coral reefs, from those 

 which have been accumulated on the floor of the open ocean. An analysis of the 

 limestone will show, in every case, whether it consists almost wholly of pure carbonates, 

 indicating that it is of coral-reef origin, or if it contains the normal proportion of silicates 

 found in all the oceanic oo^es, 



