THE ORIGIN OF IGNEOUS ROCKS. 
175 
vary considerably, and the rocks are somewhat altered, and 
contain more or less calcite. In the closely related mon- 
chiquites and fourchite there is a considerable variation in 
the alumina and alkalies, the iron and lime being high and 
the magnesia low. Fourchite is extremely high in potash, 
and in ferric oxide and lime. 
Whatever may be thought of the chemical uncertainties 
of these lamprophyric rocks, it is possible to find chemically 
equivalent rocks among the extrusive lavas. A series of 
analyses has been selected from Roth’s tables for compar¬ 
ison with those just given. They form Tables X and XI. 
The first includes leucite- and nepheline-basalt from the 
Eifel; basalt and dolerite from Meissner and Tuscany; mica- 
melaphyre from the ITartz ; mica-basalt from Arizona, and 
andesite from Mytilene. With one or two exceptions, 
these rocks are low in alumina and high in magnesia, lime, 
and iron. The alkalies are somewhat variable, but in gen¬ 
eral the potash is in excess of the soda. In the mica-mela- 
phyres the lime is low and the potash high. The excess of 
potash is particularly noticeable in the mica-basalts from 
Arizona. 
The second group includes leucite and nepheline rocks 
and hornblende-basalts, with dolerite, augite-andesite, and 
trachyte; also feldspar-basalt and limburgite from Cape Yerd 
islands. The last two exhibit a close resemblance to the 
camptonites. In general the alumina is low, the iron, lime, 
and magnesia are high, and the soda exceeds the potash, 
though in several analyses the potash is in excess. 
A study of these analyses and others which might be pro¬ 
duced makes it clear that rocks with the same chemical 
composition as the lamprophyres occur upon the surface of 
the earth as lava flows. It must therefore be the different 
crystalline development and mineralogical character as¬ 
sumed by these magmas in different modes of occurrence 
which distinguishes them from one another. This is only 
true to a certain extent, for it is evident from the mineral¬ 
ogical character of some of the lavas mentioned that they 
