90 Director's Annual Report. 



to the size and condition of the coral boulders which make up a 

 large part of its surface. 



The difhculties encountered in examining the geology of the 

 island were considerable. The density of the vegetation at times 

 made it impossible to see to anj^ distance, and at all times rendered 

 locomotion very difficult ; however, I crossed and recrossed the 

 island a number of times in every direcftion, in addition to making 

 a detailed examination of the appearance of the seaward slopes, 

 and carefully noting and photographing every important point that 

 might throw light on the island's past history. 



Turning to soundings taken by the U. S. S. Tuscarora^ across 

 the Pacific in March, 1874, we find, beginning on the i6oth 

 Meridian, 350 miles to the east of Marcus Island, that bottom 

 consisting of yellow mud was found at a depth of 3023 fathoms ; 

 at the 154th Meridian, in the vicinity of Marcus Island, the 

 sounding instrument secured coral and lava at a depth of 1499 

 fathoms; while at the i52d Meridian the bottom was found to be 

 composed of ooze at a depth of 3023 fathoms below the sea level. 

 So that from east to west Marcus Island may be regarded as the 

 summit of an elevation in the ocean's floor, ^ forming a peak or fold 

 which approaches near enough to the surface to admit of its being 

 used by the reef-building corals as a lodgment. 



The region to the west and northwest is one noted for its vol- 

 canic acftivity ; while Rota, an island north of Guam, according to 

 Quoy and Gaimard, has coral rocks upon its hills, more than 600 

 feet above the sea. The islands to the north of it are as yet little 

 known, but like Rota give evidence of change of levels ; while our 

 experience on the night on August 5tli, hereafter mentioned, leads 

 me strongly to suspect that Marcus Island is not by any means a 

 fixed island, and may yet be still further elevated. 



Having once accounted for the floor of the ocean approaching 

 near enough the surface to admit of a coral growth, the next step 

 is an easy one, for by well known laws of nature, reef -building 

 corals always tend to grow outward from a central point. The old 



•* Soundings taken by the IT. S. S. Tuscarora in the vicinity of Marcus Island: — 



Decree: 360 E. 159 E. 1.581/^ E. T.5s E. !.")() E. ir)4 E. ir.aM; E. ir,oE. 



Dppth: n02aft. 2H38ft. 2042 ft. 217-'! ft. :i27:!ft. 1499 ft. :i02:Ut. :i287ft. 



liottoui : Yel'wmud. Mud lava. Corals. CoriUs. Ooze. Cor'l lava. Ooze. Ooze. 



5 1 regret that my efforts to secure information conceroing the floor of the ocean to the 

 north and south of the island from the various cable companies who have made soundings 

 in these waters were unavailing; so that it yet remains to be ascertained whether Marcus 

 Island is simply the summit of a single subterranean mountain or a peak of a range. 



