1239 
Contact metamorphism caused by effusive rocks is eo ipso insigni- 
ficant, both on account of the low temperature at which the meta- 
morphism takes place, and on account of the escape of the pneu- 
matolytic gases during the eruption. It can however be studied in 
the enclosures of older rocks on which the magma whilst still under 
pressure, could react in the same way as if it were a deep-seated 
rock. Lacrorx') had divided the effusive rocks into two groups, accord- 
ing to the character of their contact metamorphosis: the basaltic and 
the trachytic rocks, differing from each other in either containing 
or not containing orthoclase and acid plagioclase. The metamor- 
phosis caused by rocks of the former group is chiefly restricted to 
the influence of heat confined to a narrow contactzone, whereas 
the rocks of the latter group, in consequence of their greater visco- 
sity during the effusion, and in consequence of the pneumatolytic 
gases dissolved in these viscous magmas, are in less intimate contact 
with the inclosures, but, impregnating these inclosures by pneumato- 
lytic substances can cause intensive chemical changes, which are 
not restricted to the contactzone, but can affect the entire enclosure. 
The leucitite containing the enclosures which will be now de- 
scribed, shows besides many phenocrysts of augite, numerous strongly 
resorbed phenocrysts of biotite. Macroscopically we see both the 
minerals the augite of a green, the biotite of a brownish red colour 
contrasting against a greyish-black or brownish-red ground-mass. 
The augitephenocrysts are under the microscope colourless or 
greenish; traverse-twins occur, and also twins according to (100) 
sometimes with polysynthetic lamels. Greenish and colourless por- 
tions alternate without regularity or in zones in the same crystal ; 
sometimes there is a green core surrounded by an uncoloured mar- 
gin; occasionally one sees a green band between a colourless core 
and a marginal zone, both of which extinguish simultaneously, but 
not together with the green transition-zone. Similar zones with 
varying optical properties occur likewise without observable diffe- 
rences of colour. As a rule the augite is poor in enclosures, only 
a few little prisms of apatite and flakes of biotite are enclosed. 
The biotitecrystals are for the greater part strongly resorbed; 
some are entirely altered into a black ore which can be proved to 
represent altered biotite by comparison with crystals, in which still 
remains of the strongly pleochroitie biotite can be distinguished dimly 
between the specks of the ore. There is likewise a younger genes 
ration of biotite, just as has been described by me from a micas 
1) A. Lacrorx. Etude sur le Métamorphisme de contact des roche; volcaniques. 
Mémoires préseatés par divers Savants à l'Académie des Sciences. Tome XX XI. 1894, 
81* 
