294 NEW ZEALAND NATURE-STUDY BOOK 
Quartz, Felspar, and Mica. It differs from Granite in not 
being altogether crystalline. In Granite the whole of the 
minerals composing the rock are in a crystalline form i.e. 
the rock is wholly-crystalline ; in Rhyolite a number of 
crystalline minerals are imbedded in a non-crystalline or 
vitreous ground-mass. 
Observe the light grey colour of the specimen; notice 
that it is compact, dense, and crystalline. Ascertain 
whether in structure it shows a number of small spherical 
masses (having a radiating structure when broken across) 
or well defined bands of varying colour—both the 
“spherulitic” and “banded” structures are characteristic 
of Rhyolite. 
Test for its hardness and determine its specific 
gravity ; ascertain that it fractures with an uneven 
surface and that it is without cleavage, taste, or odour. 
Norer.—It might be well, before dealing with unstratified rocks, 
to give some lessons on volcanic action and volcanoes. This 
would be specially interesting in a district within reach of 
voleanic outcrops, which should be visited and the massive 
unstratified form of the rocks made clear. It is not recommended 
that the difference between plutonic and volcanic rocks should be 
taught—it is best to call them all igneous or eruptive rocks. 
Their examination should be directed towards bringing out 
common characters—viz. that they are all close erained, compact, 
hard, without stratification, and crystalline. 
Any classification of igneous rocks must under any circumstances 
be a matter of some difficulty, so that each teacher will have to 
decide how far it is desirable to go in this direction — much will 
depend on the school surroundings, the tastes of the teacher, and 
the intelligence of the pupils. Whatever is attempted should 
have simple beginnings, and in development should avoid as far 
as possible involved and complicated details. 
It is needless to say that there will probably be other volcanic 
rocks within reach of the school, besides those described. These 
should be collected, examined, and named. A short description of 
the rock with its name and place of occurrence (locality) should 
