G2 WALDEMAR LINDGREN 
Niggli, for instance, calculates the contractions of volume of certain 
ottrelite schists’ on this basis starting with a shale of assumed 
composition. The conditions here are very complex, and I am sure 
that such computations should be undertaken with the greatest 
caution. 
Field geology has never, as far as I am aware, proved that a 
contraction of volume has taken place except by rock flowage. 
From the standpoint of microscopic investigations there is no 
evidence that replacement operates in a manner different from that 
in rocks under uniform pressure, except that under stress the 
crystals tend to become elongated in direction of minimum stress. 
In the phenomena following relaxation of pressure, such as the 
development of metacrysts of garnet and andalusite, chiastolite and 
sillimanite, we see all the familiar phases of normal replacement, 
i.e., crystal form, inclusions of matrix, and complete filling of space. 
No crystalline schist has a drusy texture or structure indicating 
contraction cracks. These metacrysts or porphyroblasts develop 
normally across the matrix? and present no indication that the 
‘‘force of crystallization” has bulged the rock mass, a most improb- 
able hypothesis considering the conditions in the deep zones. 
Only if we assume that the rocks be actually soft is any such view 
tenable. It is true that the lamellae of mica sometimes bend 
around the crystal. These phenomena have been studied by 
several authors? who have concluded that they are caused by later 
mylonitic movements on gliding planes producing rotation and also 
development of “quartz tails” on both sides of the crystal. That 
rotational movements actually have occurred has been proved 
among others by F. H. Lahee and H. Backlund. Conditions of 
stress resumed after the compact crystals had been formed by 
replacement would naturally tend to bend the elastic mica plates 
around the hard bodies just as glacial clays often form layers con- 
forming to the outline of included bowlders. 
t Beitrdge zur geol. Karte d. Schweitz, N.F., XXXVI, ror2. 
2H. Rosenbusch, Elemente der Gesteinslehre, toot, Figs. 73 and 74; C. K. Leith 
and W. J. Mead, Metamorphic Geology, 1915, Fig. ro. 
3 Paul Niggli, Beztrage zur geol. Karte d. Schweitz, N.F., XXXVI 1912; F. H. 
Lahee, ‘‘Crystalloblastic Order,’”’ etc., Jour. Geol. (1914), 500-515; H. Backlund, 
Geol. For. For. Stockholm, XL (1918), 101-203. 
