189 
unfaulted parts the converse is the case. The plane of com- 
position is thus a glidi.g plans. It has also acted as a solu- 
tion plane, for the boundaries of the lameliæ are often mark- 
ed by strings of irregular liquid and gaseous inclusions. In 
a section parallel to (010) the angle of inclination of these 
lamelle to the trace of the (001) cleavage is slightly variable 
between the limits + 23? and +26° These angles are greater 
than those for pure albite,* but as everything is so sharply 
defined I feel confidence in my measurements. 
Additional evidence of strain is afforded by the condition 
of the albite lamelle. These are in many instances bent and 
faulted. Where the latter effect has been produced the albite 
lamelle are suppressed for a short distance on either side of 
the fault line. Another striking feature is observed in the 
neighbourhood of certain quartz grains. The lamelle imme- 
diately surrounding the granules are suppressed, whilst 
those further out are curved so as to very strikingly resemble 
the “knots” in a piece of pine. The resemblance is still fur- 
ther apparent where the quartz grain does not appear in the 
thickness of the section, and only the bending of the lamellæ 
is apparent. These observations seem to suggest that in 
these rocks at all events twinning after the albite law is of 
primary origin, but that molecular shifting may be produced 
by pressure, the result beina an obliteration of the twinning. 
On the other hand, twinning after the pericline law is of 
secondary origin, and is accompanied. by the development of 
gliding planes parallel to the plane of composition of the 
twinning. 
These results are interesting, and, if borne out by further 
investigation, may be of considerable importance. I do not 
by any means wish іс announce them as a general law: the 
statement applies simply to the rock under examination. 
Quartz is fairly abandant, and is under two fairly distinct 
habits: i., small, round sections, included fairly centrally in 
the albite; ii., larger and more irregular grains, which are 
interstitial in character, or else are included in the peripheral 
portions of the felspar sections. 
The sections under the first category are, a few of them, 
certainly idiomorphic, the habit being that often found in 
quartz porphyries, viz., with almost entire suppression of the 
prism. Most of the sections are, however, nearly circular or 
only subangular. Those most centrally situated in the fel- 
spar exhibit no trace of optical continuity, but towards the 
* Rosenbusch-Iddings : < Microscopical Physiography of the Rock- 
making Minerals,” 4th edition, p. 329. 
