1917] MACCAUGHEY—HAWAIIAN FLORA 399 
occur on the leeward slopes of Haleakala and in the caldera itself; 
on the leeward slopes of Kea and Loa; and great beds to the 
leeward of Kilauea. Perhaps the largest area is in the Ka-u 
district, where, according to Hircucock,’ it covers “an area of 
300 sq. mi.”’ It is not within the province of this paper to enter 
into any detailed account of these dust deposits, but two important 
floristic relations may be enumerated: (1) the obliteration of any 
vegetation that may have occupied the region previous to the 
deposit; (2) the thick layer of ashes, if rainfall or irrigation be 
sufficient, forms a rich and mellow soil and transforms what would 
otherwise have been lava waste land into productive country. 
The plantations and ranch lands of Ka-u owe their origin to this. 
It may be pointed out, in conclusion, that similar deposits of 
volcanic ash, of great area and thickness, occur in Central America, 
Mexico, the Sierra Nevadas, the Great Basin, Utah, Montana, 
South Dakota, Nebraska, Kansas, Washington, Oregon, Alaska, 
Canada, and many other places. 
Ecological factors 
SLoPpE.—There is considerable variation in the gradient of the 
various high mountains, but on the whole it is remarkably gentle. 
Loa has a deceptively gentle slope, averaging 4-6° and not over 
8° at the steeper places. Its outline against the sky is that of a 
very much flattened dome, or “whaleback.”’ The slopes of Kea 
and Haleakala are more abrupt, usually about 12°, but sometimes 
as high as 15°. Hualalai is the steepest of the 4 mountains, par- 
ticularly near its summit, with an average slope of 14-18°. The 
cinder and ash cones have slopes which lie at the critical angle 
for material of this character, namely, 30 or 40°. The mountains 
are all relatively young and have not been carved by deep, pre- 
cipitous-walled amphitheaters of erosion, as have the mountains 
of Kauai, Oahu, eastern Molokai, and West Maui. 
PRECIPITATION.—The only comprehensive records of rainfall 
in the Hawaiian Islands are those made by the United States 
Weather Bureau and the United States Hydrographic Survey. 
The records of the former are collaborated from the reports of 
3 Hircucock, C. H., Volcanoes of the Hawaiian Islands, p. 153- 
