292 NEW ZEALAND NATURE-STUDY BOOK 
Basalt. Observe the general shape and colour of the 
specimen. Notice that, although it appears crystalline, the 
individual crystals cannot be made out as in Granite. 
Basalt may be called a crystalline rock, though it differs 
from Granite in not being wholly-crystalline ; this difference 
in structure, however, cannot be seen without the aid of 
the microscope. 
~The texture of basalt varies with the specimen ; notice 
whether this is close-grained and compact, or vesicular 1.¢. 
containing small cavities in greater or less abundance. 
Many volcanic rocks show this structure —at the time of 
their eruption they were saturated with steam, and the 
expansion of the imprisoned vapour has subsequently de- 
veloped internal cavities. 
Test for its hardness and compare with Granite. 
Notice that the typical Basalt is a hard close-grained 
compact rock, tough and durable — qualities which render 
it valuable as building-stone and as road-metal. 
Observe its want of definite cleavage, its uneven fracture 
and the absence of taste and odour. Ascertain its specific 
gravity and compare with other rocks. 
Basalt occurs in several parts of New Zealand in some 
places forming rock-masses of considerable extent. It is a 
voleanic rock and has been poured out at the surface in 
the form of lava flows. These frequently assume what is 
known as a columnar structure—that is the mass splits into 
slabs or columns, each having a varying number of sides. 
An exposure where this has occurred shows a mass of 
closely fitting columns, in some cases arranged with 
wonderful regularity. 
Uses. Basalt is used as a building stone and as road 
metal, its hardness, durability, and property of resisting 
the attacks of the atmosphere and the weather, rendering 
it valuable for these purposes. 
