17G 
IDDINGS. 
are closety related in mineral composition to the dike rocks 
in question. Examples of these are the hornblende-basalt, 
mica-melaphyres, and mica-basalts, besides rocks which 
Eosenbusch has described as having camptonitic and mi- 
nette-like characters.* 
There are some, however, in which the mineralogical 
difference is essential, the one group being characterized by 
abundant mica, the other being free from it, and having 
certain other distinguishing features; but this distinction 
does not depend simply on the occurrence of the magma in 
dikes, for it is well known that dikes of all of the volcanic 
lavas may be found, the rocks being almost, if not abso¬ 
lutely, identical in both cases; consequently magmas that 
are chemically the same as those of the so-called “ gang- 
gesteine ” occur in dikes, but have crystallized into rocks 
which differ from “ ganggesteine ” to .as great an extent as 
any of the surface flows of the same magmas; hence it is 
not the occurrence of these rocks in dikes which has ren¬ 
dered them unusual, but it is the existences of certain 
physical conditions at the time of their crystallization which 
led to the development of particular mineral associations. 
In the great majority of cases the lamprophyres are charac¬ 
terized by the presence of biotite, sometimes with orthoclase, at 
others with plagioclase, and occasionally with olivine. The 
extrusive equivalents are generally rich in olivine, with no 
biotite, and in many cases carry leucite and nepheline or sani- 
dine and alkali plagioclase. From the nature of the theoret¬ 
ical molecule of biotite, which is (K, H) 2 0, (Al, Fe) 2 0 3 , 
2 $i0 2 -f 12 (Mg. Fe) 0, 6 $i0 2 , it is evident that if the silica 
of a magma is too low to convert all the alkali-alumina 
molecules into silicates, with the ratio 1:1:6, and the 
magma is rich in magnesia and potash and low in alumina, 
it may split up into olivine crystals, n [2 (Mg, Fe) 0, Si0 2 ], 
and leucite crystals, m [K 2 0, A1 2 0 3 , 4 Si0 2 ], besides an 
* Nepheline rocks with camptonitic habit (Mikro. Phys. d. mas. gest. 8°. 
Stuttgart, 1887, vol. 2, p. 795). Hornblende-basalts (ibid., p. 738). Mica- 
trachyte of Monte Catini, Tuscany, an equivalent of minette (ibid., p. 597). 
