148 GEOGNOSY. [Box IL. 
Four varieties are distinguished according to texture: dolerite, 
anamesite, basalt, and vitreous basalt. 
Dolerite (greenstone, in part, of older authors). This includes 
all the larger-grained kinds in which the component crystals can be 
readily distinguished with the naked eye. The felspar, which 
among the basalt-rocks is probably often a more silicated form than 
labradorite, is usually the most conspicuous ingredient, the dark 
prisms of augite, and the dusty or minutely octahedral magnetite 
give the grey .or black hue to the rock. The microscopic structure 
is crystalline, though a small quantity of an amorphous base may 
here and there be traced (Fig. 11). | 
Anamesite includes those kinds of which the texture is so 
fine that the naked eye can observe only that the mass is a finely 
crystallised granular aggregate. Under the microscope more of an 
amorphous base with microliths is seen than in dolerite. 
Basalt.—This name when used as the designation of a 
particular rock is applied to those black, extremely compact, 
apparently homogeneous varieties which break with a splintery or 
conchoidal fracture. The component minerals can only be observed 
with the microscope, unless where they are scattered porphyritically 
through the mass. ‘They consist of those above mentioned, and 
between them may be traced .a base which is sometimes a glass, 
but is often partially devitrified by the appearance of various 
crystallites. ‘These sometimes so increase that the glass disappears, 
and its place is taken by an aggregate of minute granules, hairs, 

Fic. 25.—Juncrion or intrustvE DoLeriTe witn Sanpsrong, SALIspury Crac, 
EprnsurcH. MAGnNiriep 20 Diameters. 
The granular portion at the bottom of the drawing is Sandstone, a part of which ia 
involved in the Dolerite that occupies the rest of the slide. The darker portion 
next the Sandstone is a vitreous substance which has ‘been serpentinized. It 
contains crystals of Plagioclase and vapour vesicles drawn out in the direction of — 
flow. Above the darker part the glassy condition rapidly passes into ordinary but 
minutely crystalline Dolerite. The rock ‘has been considerably altered, calcite 
occupying many of the vesicles and fissures, 
needles and crystals. The proportion of this base varies within 
wide limits, insomuch that while in some basalts it so preponderates 

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