504 
ELEMENTS OF GEOLOGY. 
solutely fixed as to their kind and quality; but one ingredi¬ 
ent may be replaced by an equivalent portion of some anal¬ 
ogous ingredient. Thus, in augite, the lime may be in part 
replaced by portions of protoxide of iron, or of manganese, 
while the form of the crystal, and the angle of its cleav¬ 
age planes, remain the same. These vicarious substitutions, 
however, of particular elements can not exceed certain de¬ 
fined limits. 
Basaltic Rocks. —The two principal families of trappean or 
volcanic rocks are the basalts and the trachytes, which differ 
chiefly from each other in the quantity of silica which they 
contain. The basaltic rocks are comparatively poor in silica, 
containing less than 50 per cent, of that mineral, and none 
in a pure state or as free quartz, apart from the rest of the 
matrix. They contain a larger proportion of lime and mag¬ 
nesia than the trachytes, so that they are heavier, independ¬ 
ently of the frequent presence of the oxides of iron which 
in some cases forms more than a fourth part of the whole 
mass. Abich has; therefore, proposed that we should weigh 
these rocks, in order to appreciate their composition in cases 
where it is impossible to separate their component minerals. 
Thus, basalt from Staffa, containing 47*80 per cent, of silica, 
has a specific gravity of 2*95; whereas trachyte, which has 
66 per cent, of silica, has a sp. gr. of only 2*68; trachytic 
porphyry, containing 69 per cent, of silica, a sp. gr. of only 
2*58. If we then take a rock of intermediate composition, 
such as that prevailing in the Peak of Teneriffe, which Abich 
calls Trachyte-dolerite, its proportion of silica being inter¬ 
mediate, or 58 per cent., it weighs 2*78, or more than tra¬ 
chyte, and less than basalt.* 
Basalt, — The different varieties of this rock are distin¬ 
guished by the names of basalts, anamezites, and dolerites, 
names which, however, only denote differences in texture 
without implying any difference in mineral or chemical com¬ 
position : the term Basalt being used only when the rock is 
compact, amorphous, and often semi-vitreous in texture, and 
when it breaks with a perfect conchoidal fracture; when, 
however, it is uniformly crystalline in appearance, yet very 
close-grained, the name Anamesite (from avafxecTOQ^ intermedi¬ 
ate) is employed, but if the rock be so coarsely crystallized 
that its different mineral constituents can be easily recog¬ 
nized by the eye, it is called Bolerite (from hoXepog^ deceitful), 
in allusion to the difficulty of distinguishing it from some of 
the rocks known as plutonic. 
Melaphyre is often quite undistinguishable in external ap- 
* Dr. Daubeny on Volcanoes, 2d ed., pp. 14, 15. 
