Notices of Memoirs—Prof. Carvill Lewis— Matrix of Diamond. 129 
a third series of widely-distributed gravels, which are believed to 
indicate a more extensive and an older advance of the glaciers. 
The character and origin of the lakes not only of the Salzach 
district, but of the Linth and those of the Neuenburg group, are 
fully treated, and a special chapter is devoted to the Glacial deposits 
of the Lake of Geneva. 
The text is accompanied by several figures and tables, as well as 
by three elaborate coloured maps of the district described. 
Il.—Tus Matrix or tae Dramonp. By Pror. Carvitt Lewis. 
(Abstract of a Paper read at the Manchester Meeting of the British Association, 
September, 1887.) 
MICROSCOPICAL study of the remarkable porphyritic peri- 
dotite which contains the diamonds in South Africa demon- 
strates several interesting and peculiar features. 
The olivine, forming much the most abundant constituent, is in 
porphyritic crystals, sometimes well bounded by crystal faces, at 
other times rounded and with corrosive cavities, such as occur in it 
in basaltic rocks. It rarely encloses rounded grains of glassy bronzite, 
as has been observed in meteorites. The olivine alters either into 
serpentine in the ordinary way, or into an aggregate of acicular 
tremolite crystals, the so-called ‘pilit,’ or becomes surrounded by a zone 
of indigo blue bastite—a new variety of that substance. The olivine 
is distinguished by an unusually good cleavage in two directions. 
Bronzite, chrome diallage, and smaragdite occur in fine green 
plates, closely resembling one another. The bronzite is often 
surrounded by a remarkable zone, with a centric pegmatitic, or 
chondritic structure, such as occurs in certain meteorites. This 
zone is mainly composed of wormlike olivine grains, but a mineral 
having the optical characters of cyanite also occurs in this zone. 
Biotite, a characteristic constituent, occurs in conspicuous plates, 
often twinned, generally rounded, and distinguished by its weak 
pleochroism, a character peculiar to the biotite of ultra-basic eruptive 
rocks. It alters by decomposition into the so-called Vaalite. 
Perowskite occurs in very numerous but small crystals, which 
optically appear to be compound rhombic twins. 
Pyrope is abundant in rounded red grains. Titanic iron, chromic 
iron, and some fifteen other minerals were also found. Rutile is 
formed as a secondary mineral through the alteration of olivine into 
serpentine, being a genesis of rutile not heretofore observed. 
The chemical composition shows this to be one of the most basic 
rocks known, and is a composition which by calculation would 
belong to a rock composed of equal parts of olivine and serpentine, 
impregnated by calcite. 
The structure is at the same time porphyritic and brecciated, being 
one characteristic of a volcanic rock which after becoming hard had 
been subjected to mechanical movements. It is a volcanic breccia, 
but not an ash or tuff, the peculiar structure being apparently due 
DECADE III.—VOL. V.—NO. III. 9 
