330 
Interpretation of the structure. — The fact, that when abys- 
sal or hypabyssal rocks are considered the size of the grains is 
independent of depth, has been demonstrated at other localities’. 
Many instances are also known where a coarse-grained rock 
has crystallized nearer the surface of the earth than a less 
coarse-grained one”, The usual explanation given is that the 
size of grains depends upon the chemical composition of the 
magma. An explanation of this kind, however, is not applicable 
to the structure of the lujavrite of Ilimausak. Firstly, it does 
not explain the schistosity of the rock, and secondly, it appears 
to be quite insufficient to account for the small size of grains. 
Not only is there a vast contrast between the size of grains of the 
lujavrite and those of the superincumbent rocks, but the lujavrite 
itself contains an intercalated rock body of kakortokite which 
has the same chemical composition, but is much coarser grained. 
Moreover the fine-grained texture seems to be related to the 
schistosity: where the schistose structure is wanting the rock is 
somewhat coarser. 
A key to the explanation both of the small size of grains 
and of the primary schistosity of the Greenlandic lujavrite may 
probably be found in the fact (see p. 324), that during the cry- 
stallization of the lujavrite the overlying strata have sunk; as 
far as that part of the rock which envelops the naujaite frag- 
ments of the breccia zone (p. 322) is considered, it is obvious 
that there must be a connection between the magmatic move- 
ments and the structure of the lujavrite. On the other hand, 
there is no perceptible difference in structure between the 
lujavrite of the breccia zone and the bulk of the rock, but that 
the parallel arrangement of the mineral components in the 
т W. Cross, The Laccolitic Mountain Groups of Colorado, Utah, and 
Arizona. U. S. Geol. Survey, 14th Annual Report, Part Il, 1894, р. 230. 
? WH. Weep and Г. У. Pirsson, Geology of the Judith Mountains. U.S. 
Geol. Survey, 18th Annual Report, Part III, 1898, p. 574; Geology of the 
Little Belt Mountains, ibidem 20th Ann. Rep., Part HI, 1900, р. 562. 
