176 PROFESSOR J. W. JUDD. 



great chemical and mineralogical changes may go on in coral-reef limestones, 

 there must always be some element of uncertainty in fixing upon the sequence of 

 rocks in an upraised coral reef such as does not exist in the case of a vertical bore- 

 hole. 



It is only fair to point out that in his recent valuable researches on tlie upraised 

 reefs of the Pacific, Professor Agassiz lias taken every possible precaution to avoid 

 these obvious sources of error. Wherever possible, the specimens collected were 

 obtained in deep ravines, and by means of blasting ; but even when all this was 

 done, the series of specimens collected must have been very far from constituting a 

 vertical sequence, like those from a bore-hole. 



B. — The Materials from the Lagoon of Funafuti. 



The specimens collected from the lagoon of Funafuti are of scarcely less interest 

 and importance than those obtained from the several borings in the islets of the 

 atoll. The lagoon of Funafuti Jias an extreme length from north to south of 

 14 miles, and a breadth from east to west of 9 miles. Its depth varies from 12 to 

 30 fathoms, with a pro})able average of 2U fathoms. There are wide and deep 

 openings between the islets encircling the atoll, especially on the south-east and 

 north-west sides, (consequently this body of salt water (whicli is as deep as the 

 Straits of Dover) is in free comnumication witli tlie open ocean, and, thougli affording 

 a safe anchorage ground, is by no means a mass of still or dead water. Tbe com- 

 munication with the open ocean is so free tliat, contrary to what is the case in the 

 neighbouring atoll of Nukulailai, the waters of the lagoon flow freely in and out at 

 the change of the tides, and there is never any difference of level between the lagoon 

 and the outer ocean. The inflowing and outflowing waters probably find their way, 

 not only through the wide and deep channels between the islets, but to some extent 

 through the body of the reef, wliicli in its upper portions would appear to be almost 

 as open and pervious as a sponge. 



Besides a iiumber of sporadic dredgings within its area, we have tlie results, a 

 very systematic examination of the floor of the Funafuti Lagoon. Mr. G. H. Halligan 

 and Mr. A. E. Finckh made a series of dredgings with the sand-pump at distances 

 of -^ mile from a point opposite to the Mission (Jhurch at Fongafale (Funafuti 

 Island) in a direction slightly north of west to Fuafutu Island. (See Plate 1.) The 

 former gentleman, with the aid of Commander Sturdee, li.N., and of the officers 

 and men of H.M.S. " Porpoise," was able to put down two borings in the lagoon, 

 about 1^ miles west of the Mission Church, and slightly north of the line of dredgings. 

 The depth of the lagoon where the boring was made was 101 feet, and the deepest 

 of the borings was carried to 245 feet from low- water mark spring tides, or 144 feet 

 below the floor of the lagoon. 



The dredgings in the lagoon showed that except where bosses of coral lock rise 



