UNSTRATIFIED OR IGNEOUS ROCKS 293 
The soil resulting from the weathering of Basalt is 
usually fertile and easily worked. 
ANDESITE. 
Under this term are included volcanic rocks com- 
posed of Felspar, Augite, and other minerals. The 
rock itself differs from Basalt mainly in the absence of 
Olivine. Some varieties of Andesite show large well 
defined crystals of Augite or Hornblende and, according as 
one or other of these minerals predominates, are known as 
Augite-andesite or Hornblende-andesite. 
The rock itself is very like Basalt in appearance ; indeed 
it requires some expert knowledge to distinguish between 
average hand specimens. Those kinds which show well- 
marked Augite or Hornblende crystals are easily identified. 
Examine the specimens, as in the case of Basalt, and 
ascertain that Andesite is a compact close-grained 
rock, relatively free from vesicles, crystalline, and 
dense, that it is tough and hard, breaks with an 
uneven fracture, and is without taste or odour. 
Certain kinds of Andesite, when beginning to weather, 
have a tendency to cleave along definite lines—the rock 
tending to break up into rectangular masses of varying 
size. This property is rarely met with in fresh un- 
weathered exposures—the typical Andesite showing no 
cleavage. 
Determine its specific gravity and compare with other 
rocks. Andesite occurs in most countries, and is amongst 
the best known of New Zealand volcanic rocks—being 
found throughout both islands. It is used for building 
purposes and for road metal. 
RHYOLITE. 
Rhyolite is a voleanie rock almost identical in com posi- 
tion with Granite—that is, its essential constituents are 
