ECOLOGICAL CHARACTERISTICS OF MARIANA ISLANDS. ff 
mountains descend rapidly; the eastern faces are equally abrupt at first and then 
become more gradual as they merge into the lower slopes extending to the eastern 
shores. | 
Although the range of higher land is virtually continuous, three constituent 
parts can be distinguished. The northern portion lies west of the narrow median 
neck of the whole island, and its lower levels are composed largely of limestone; 
the high points are Mount Chachao, 1,046 feet, Alutom, 1,080 feet, and Tenjo, 
1,013 feet. South of this group of peaks the terrain is considerably lower, and at 
present it is so barren as to constitute a barrier to the crossing of snails from the 
northern group of uplands, inhabited in part by the animals, to the middle section 
of the range, or vice versa. Beginning with Mount Salifan, 858 feet high, the median 
section of the range extends south to Alamagosa and Lamlam, 1,334 feet, Jumullong 
Manglo, 1,274 feet, and so on to Bolanos, 1,240 feet, which lies directly east of the 
village of Umatac (plate 3, B and cc). Only a slight depression intervenes between 
Bolanos and the smaller group of mountains that constitute the third and southern- 
most section of the entire range; the main peaks of this last division are Mount 
Schroeder, 1,054 feet, and Sasalaguan, 1,110 feet high (plate 3 c). 
The limestone strata of the southern half are continuous with those of the 
north, and they conform also in their inclinations. Hence as the observer passes 
from north to south along the western coast, the uplifted reef rock gradually de- 
creases in height, until at Merizo, at the southwestern extreme, it becomes virtually 
continuous with the living reef beyond the shore. As in the north, so in this part 
of Guam, the limestone cliffs are high on the eastern side, but passing inland, the 
rock of the lofty coastal bluffs disappears beneath the overlying materials washed 
down from the volcanic masses of the principal range of mountains. 
Rainfall is quite heavy, but there are no constant streams in the northern part 
of Guam, owing to the receptive nature of the rock and its internal drainage. In 
the southern volcanic portion, many streams exist which flow with brief courses on 
the western side, while those upon the eastern slopes are necessarily longer. The 
latter run between the relatively abrupt bounds which have been cut by earlier 
erosive action through the tuffs and limestone rock of their territory. The physio- 
graphic features thus outlined are most important, because the areas of vegetation 
that are suitable for Partule are directly conditioned by them. 
Inasmuch as collections of Partula gibba were taken from several localities on 
the western side of Saipan, the general geological nature of that island requires brief 
consideration. Saipan consists largely of coral limestone surrounding a volcanic 
base. In topographic respects it is a very simple unitary mass, rising to a single high 
peak, Mount Tapochau, about 1,530 feet in height (plate 8 a); the slopes descend 
gradually and evenly, and are not grooved by large and deep valleys and gullies. 
Along the western side, from Tanapag to Agingan at the southwestern point, there 
is a flat coastal plain which attains a maximum radial width of more than a mile; 
but elsewhere the borders of the island are relatively abrupt as they reach the sea. 
Hence in general contour, Saipan is much more like the southern than the northern 
part of Guam, although its constituent rock is so largely calcareous. Only the high 
