Mlarvels of the Untverse 1055 
name of Granophyre is given. Such granite is 
found in the Island of Skye, and the fracture has 
facets which resemble characters in old Hebrew 
writing. 
When a portion of the molten magma, from 
which the granite is eventually formed, manages 
to escape by a vent and cause an eruption of 
acid lava, the composition of the rocks thus 
formed may be identical with the granite, and 
yet their structures are completely different. In 
the volcanic neck itself, the lava will take the 
longest time to solidify, but although cooled 
slowly, it is not now under the great pressure 
under which granite is formed, and the structure 
is called micro-crystalline or porphyritic. This 
is a ground-mass of such small crystals that 
only the higher powers of the microscope reveal 
it as different from a glass, and in this there is 
only a small proportion of the felspar and quartz 
separated as complete crystals. This quartz 
and felspar Porphyry occurs as long veins or 
dykes branching out from the Cheviot Hills, and 
similar dykes are called Elvans by the Cornish 
miners. 
If the acid volcanic lava has been poured 
out in sufficient quantity on the older surround- 
ing rocks, it will solidify less gradually than in 
the vent, and before the flow has been completely 
stopped the first-formed crystals render the mass 
pasty. The still fluid portions drag past these 
crystals until they become solidified as a glass 
with a characteristic fluidal structure. This 
formation is called Rhyolite, and is well marked 
in the lava flows of Iceland, which exhibit the 
differently-coloured glassy bands flowing round 
the numerous minute crystals of felspar and 
quartz, and carrying along in themselves crystal- 
lites of other minerals. 
As the speed of the lava flow becomes less 
and less, this flow structure gradually  dis- 
appears, and the crystallization begins at a 
nucleus, which may be a _ small particle of 
magnetite, and extends radially in all directions. 
This forms a number of Spherulites, which are 
simply segregations into spots of partially- 
devitrified glass possessing a different colour 
from the surrounding matrix. This structure, 
known as Spherulitic Rhyolite, is so highly 
developed in some of the Jersey rocks that the 
OBSIDIAN. 
From Iceland, showing microlites of felspar (white), and 
elongated spherules of brown glass. Magnified 20 times. 
Photos by] ([P. Primrose. 
OOLITIC LIMESTONE. 
From Shropshire, showing shells and rounded grains of 
calcite. Also called Roe-stone. Magnified 40 times. 
