Notices of Memorirs—Dr. Scott—The Arran Pitchstones. 469 
large proportion of liparite present results from its lesser resistance 
to ice-erosion compared with basalt, whereby the huge cirque has 
been excavated where the acid rocks occur, and the material deposited 
to form the present remarkable stream. It has also been objected 
that none of the blocks are ice-scratched, but this is not to be 
expected owing to the exceptional fissility of liparite and its rapid 
degradation under weathering influences. The author has never 
seen an ice-scratched boulder in Iceland. 
IV.—Tuer Perrotocy or THE ARRAN Pircusrones. By ALEXANDER 
Scorr, M.A., D.Sc. 
LTHOUGH the Arran pitchstones are so widely known, no 
extensive examination of them has ever been made. The 
intrusions, which number about eighty, may be divided into the 
following groups :— 
1. Non-porphyritic glasses with abundant riierolites which are 
generally hornblende. These are found chiefly in the district 
round the coast, and include the Corriegills and Monamore Glen 
occurrences. 
2. Pitchstone porphyries with large phenocrysts of quartz and 
felspar and scarce augite, and with hornblende microlites. This 
group includes many of the dyke-rocks intrusive into the Goatfell 
granite. 
_ 3. Pitchstone porphyries with aibans crete of felspar and pyroxene 
and subordinate quartz. The pyroxene includes both augite and 
enstatite, and scarce crystals of an iron-rich olivine also are found. - 
Microlites of pyroxene and of hornblende occur. This group is typical 
of the intrusions of the south end of the island. 
4. More basic type with scarce phenocrysts and great abundance of 
pyroxene microlites. This group is represented by two occurrences 
in Glen Cloy and several around the great Tertiary volcanic vent. 
Analyses have been made of each type, and the results show the 
existence of considerable variation in composition. An attempt has 
been made to determine the cooling histories from the examination of 
the field-relations and microscopic structures of the various types, 
and also to indicate the conditions which are responsible for such 
a large development of glassy intrusive rocks. 
REVIEWS. 
I.—Tup Larpr Staces or tHe Evoturion or tHE Jenrous Rocks. 
By N. L. Bowen. Journ. Geol., Supp. to vol. xxiii. pp. 91. 
1915. 
fY\HE determination of the processes whereby the various types of 
igneous rock have been derived by differentiation from a few 
relatively simple magmas has long been, and still is, one of the chief 
problems of petrology. Of the hypotheses which have been advanced 
in order to explain differentiation, several have been discredited and 
are seldom advocated. nowadays; thus ‘‘Soret’s principle’’, to which 
