1917] MACCAUGHEY—HAWAIIAN FLORA 407 
heaviest leather boots. Horses, cattle, and goats use the pa-hoe-hoe 
as natural roadways but refuse to cross the a-a. It is due to this 
impassability of the a-a that plants growing upon it, or in areas 
surrounded by it, are protected from the devastations of ‘-herbivo- 
rous animals. Thus portions of the primitive flora have been 
preserved in regions where they would have otherwise been de- 
Fic. 18.—Front or end of 1917 flow, which issued from flanks of Loa, composed of 
a-—a, flowing over older pa-hoe-hoe; smoke arising from various portions of flow; note 
; mR ‘ i . 2. AN Sap OS ME 
angular fragments of which a-a is composed, tree at lower left is Metrosideros poly 
mor pha. 
stroyed by wild cattle and goats. A-a lacks the glistening appear- 
ance of pa-hoe-hoe and is usually dark chocolate brown instead of 
black. Sometimes the reddish tint is quite pronounced. The 
brown and red are due to the high iron content, which characterizes 
all the Hawaiian lavas. 
CHEMICAL COMPOSITION OF LAVA.—The following table illus- 
trates the chemical composition of a typical Hawaiian basaltic 
