PRELIMINARY REPORT. 39 
Guam, a depth 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.? 
I 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 coralliferous limestone. The vertical cliffs bor- 
dering the eastern face rise toa 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, Niue, Eua, Vavau, and others in the. Fijis, which 
had made their cliffs a familiar feature in our explorations. In fact, out- 
side of Viti Levu 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 
voleanic, 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 Agafia, 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 Agafia 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 voleanic 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 
1 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 “Challenger” to the 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, VIIL, p. 180. 
