6 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
Guam, Tinian, and Saipan are the only islands which are reported as the 
habitations of Partule, and therefore we have no real concern with the smaller 
members of the group to the northward. As stated above, field-work was carried 
out in Guam and Saipan; it was impossible to visit Rota and Tinian, despite their 
seeming proximity, owing to unsettled and dangerous typhoon weather and to the 
insuperable difficulties of transportation. 
All of the islands of the group are volcanic in origin, in whole or in part. Some 
of the lesser elements to the north are almost perfect single cones, while others are 
multiple in nature; their recent construction is indicated by continued eruptions 
from their craters as well as by their simple uneroded contours. The height exceeds 
2,000 feet in some instances, such as Alamagan. The age of the islands is regularly 
greater as one passes southward; Saipan is clearly a much older element than the 
northernmost islands, while Guam 1s a product of a still earlier period of construc- 
tion and the most ancient of them all. Limestone forms a large part of the southern 
land-masses, which comprise wide strata of such materials now raised high above 
the sea. Living coral reefs fringe the coasts of some of the southern islands, 
mostly on their western sides, but such reefs do not occur about the northern 
elements. 
The island of Guam has a length of about 29 statute miles on a line running 
from south-southwest to north-northeast—a direction that coincides very closely 
with that of the line passing to Rota, Tinian, and Saipan (plate 1). At its center, 
Guam is only 4 miles across, while in the wider areas to the north and south the 
breadth varies from 7 to 9 miles. 
Although they are so directly continuous, the two main portions of Guam differ 
markedly in geological respects and consequently in ecological conditions. Viewed 
from a distance, the northern (or northeastern) half appears to be an almost flat 
plateau, raised from 200 to 500 feet above sea-level (plate 2, a and B). On closer 
study, this part proves to be composed of elevated reef-limestone, very much worn 
and sculptured, through which volcanic masses have broken their way, as attested 
by the metamorphosis of the contiguous calcareous rock. Barrigarda is such an 
extruded mass, 674 feet in height, near the geographical center of the island; Santa 
Rosa, of the same nature, reaches a height of 870 feet, and is situated about 6 
miles to the east-northeast of Barrigarda. Elsewhere in the northern half of Guam 
only a few hillocks rise decidedly above the general level of the plateau. The 
elevated reefs are not simple, however, for investigation discovers at least five 
component strata, tilted somewhat from north to south and from east to west. 
In summary, this part of Guam consists mainly of a series of limestone terraces, 
not exactly level, which have been raised by a succession of uplifts alternating with 
periods of quiescence. 
The southern half of Guam is very different, as it is primarily a volcanic massif 
with restricted calcareous strata around its base. A somewhat irregular range of 
mountains runs more or less parallel to the west coast (plate 1 and plate 3, a, B, 
and c), with the highest points from 1 to 2.5 miles back from the shore, and from 
700 to more than 1,300 feet of altitude. Necessarily the westward slopes of these 
