MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 431 
distinguish a variety (see Figs. 7 and 8) with large orbs of a diameter 
from about 4 to 8 or 9, mostly about 5 cm., and another variety with 
small orbs, of a diameter of about 2cm. These orbs consist in 
the interior exclusively of hypersthene (0.28 FeSiO,:0.72 MgSiO,, 
if we leave out of consideration the links containing CaO and 
Fic. 7.—Orbicular quartz-norite from Romsaas, Norway 
Al.O,) which, however, has been changed in part to hornblende. 
In the exterior part appears also some biotite which partly lies as 
shells or scales on the outside of the orbs (see Fig. 8). According 
to the detailed calculation which I effected on the spot, the rock 
with the large orbs consists (according to weight) of 70 per cent of 
orb substance (analysis No. 44) and 30 per cent of intervening - 
mass (analysis No. 45). On this basis, the composition, No. 46, 
of the entire rock is calculated by myself. The close conformity 
