86 E. H. L. SCHWARZ 
lusite and to show how closely the phenomena exhibited by the 
larger masses, the igneous dykes and laccolites, are presented by 
the individual crystals. 
THE DEVELOPMENT OF ANDALUSITE CRYSTALS 
The development of crystals of andalusite, chiastolite, cor- 
dierite, and staurolite is most beautifully illustrated in the rocks 
round the Great Transvaal laccolite. The metamorphic rocks 
have been described by Hall* and I am greatly indebted to him 
for supplying me with material for study from this locality. The 
andalusite substance in the less advanced types settles as irregular 
patches in the shales, schists, sandstones, or whatever type of 
sedimentary rock is becoming metamorphosed. The minerals 
of the original rock are, as it were, pushed aside to make way for 
the introduction of the new material and no absorption is at first 
apparent. In more advanced types the minerals of the original 
rock are seen to be whittled away; usually there is an outer zone 
where the andalusite crystal is more or less free from inclusions. 
The developing crystals all tend to form in about the same dimen- 
sions in the particular rock-type and it is a constant feature that the 
prism zone early asserts itself, the planes bounding the long axis 
of the crystal being quite sharp, whereas the basal planes are 
indefinite, at any rate in the Transvaal specimens that I have seen 
and in those described by Hall. The Transvaal andalusites appar- 
ently do not go beyond this stage, but the smaller variety of the 
substance, chiastolite, does form very definite, sharply bounded 
crystals. The section illustrated by Hall (Survey Report, Part I, 
Fig. 1) shows the developing chiastolite with the center still full of 
inclusions, the outer rim clear, and the complete crystals in which 
all but a small string of inclusions in the center has been cleared 
away. 
For the later stages of the development of andalusite I will take 
the andalusite schist of George.? The occurrence is on the Zwart 
tA. L. Hall, “The Geology of the Haenertsburg Gold Fields,” Report Geol. 
Survey, 1907, Pretoria (1908), 43 ff.; ‘‘Contact Metamorphism in the Western 
Transvaal,” Trans. Geol. Soc. S. Africa, Vol. XII, Johannesburg (1910), 1109. 
? E. H. L. Schwarz, ‘‘The Andalusite Schist of George,’ Albany Mus. Records, 
II, 164 (1907), Grahamstown. 
