PRELIMINARY REPORT. 39 



Guam, a ck'pth of 4813 fathoms, a depth surpassed only, if I am not in 

 error, by three soundings made by the " Penguin " in the deep trough 

 extending from Tonga to the Kermadecs, and by two still deeper made 

 by the U. S. S. "Nero"' to the eastward of Guam, probably in con- 

 tinuation of the same "Deep" called the "Caroline Deep" by Professor 

 Supan.^ 



1 was very much surprised, in approaching Guam from the eastward, 

 to find that the island was not wholly volcanic, but that the northern 

 half consists of elevated corallilerous limestone. The vertical cliffs bor- 

 dering the eastern face rise to a height of 100 to 250 or 300 feet at the 

 northern extremity, and resemble in every way those of similar limestone 

 islands, such as Makatea, Nine, Eua, Vavau, and others in the Fijis, which 

 had made their cliffs a familiar feature in our explorations. In fact, out- 

 side of Viti Lovu and Vanua Levu, this is the largest island known to 

 me where we find a combination of volcanic rocks and of elevated coral- 

 liferous limestone. The massif forming the southern half of the island is 

 volcanic, and the highest ridge, rising to about 1000 feet, runs parallel 

 to the west coast, the longest slope being toward the east. 



This volcanic mass has burst through the limestone near Agana, and 

 the outer western extension of the coralliferous limestone exists only in 

 the shape of a few spurs running out from the volcanic mass, the largest 

 of which are those forming the port of San Luis d'Apra. These spurs 

 are separated by lower ridges of volcanic rocks extending to the sea from 

 the main central mass. To the north of Agaiia the limestone forms an 

 immense irregular mesa, cut by deep crevasses, full of pot-holes and 

 sinks, rising gradually northward to a height of 350 or 400 feet. Near 

 the northern extremity of the island a volcanic mass, Mt. Santa Rosa, 

 has burst through the limestone and rises about 150 feet above the 

 general level of that part of the island. The shore stratification of the 

 bluffs is much distorted in the vicinity of that volcanic outburst. 



The formation of huge masses of limestone in which occur at intervals 



' The deep soundings of the "Nero" have been marked "Nero Deep " in H. O. Chart No. 529, 

 published June, 1900, but it does not give the deep sounding of the " Cliallenger" to tlie westward 

 (4475 fathoms), which was the first indication of the probable existence of a deep trough south of Guam. 



2 Petermann's Mitteil., 1899, Vol. 45, VIII., p. 180. 



