TRANSACTIONS OF SEGTION (©. 409 
olivine without the enormous pressure and heat used by other experimenters. 
He concludes with the opinion that a very deep-seated ultra-basic zone in which 
garnet and ferro-magnesian silicates predominate saw the crystallisation of 
diamond. 
6. On the Geology of the West Rand. By Dr. J. T. Carrick. 
The results of a survey of the West Rand, made by the author in 1899, agree 
in the main with Dr. Hatch’s geological map of the Southern Transvaal published 
in 1897. In the author’s opinion it is not yet certain that the granite to the north of 
Johannesburg is all of one age. The schists belong to the horizons respectively 
older and younger than the base of the Witwatersrand system. The author con- 
siders that the Botha’s and Main Reef series are identical. 
7. The Plutonic Rocks and their Relations with the Crystalline Schists and 
other Formations. By F. P. MENNELL, 
In this paper the term ‘plutonic’ is restricted to those vast and structurally 
uniform masses of igneous material whose superficial extent is matched by their 
extension in depth. No mere coarsely crystalline dyke comes under this head, and 
it may even be doubted whether any rock but granite strictly fulfils the above 
conditions. Such rocks will have consolidated not so much from intrusion into the 
cool upper zones of the earth’s crust, but because the liquid condition is a state of 
unstable equilibrium at the normal depth-temperature, and cannot survive the 
cessation or reversal of such crust movements as have temporarily given rise to 
fusion. Despite these restrictions plutonic rocks are extremely common in South 
Africa, nearly all being of later date than our uppermost Archean rocks, but 
older than the lowest of the later sediments. The geologist is able to examine the 
very roots of the great mountain chains, of which the granites must once have 
formed a core, probably at the beginning of the Paleozoic period. 
Tetrological Problems.—(i) Composition.—It quickly becomes obvious that 
granite, instead of being an extreme of the igneous series, represents almost exactly 
the average composition of the igneous rocks (see ‘Geol. Mag.,’ June 1904, p. 263), 
The conclusions drawn from actual observation in such an area as Rhodesia, chiefly 
occupied by igneous rocks, do not at all tally with inferences based merely on 
laboratory work, which do not allow for bulk—teally the essential feature of the 
problem. 
(ii) Causes of Heterogeneity —But though we find that granite, which is 
practically uniform over enormous areas (often many thousand square miles, in 
South Africa), is by far the most important igneous product, we have still to 
account for the fact that other rocks are met with which differ widely both among 
themselves and from the grauitic average. Appearances point to some genetic 
relation between even very divergent types, and it has been suggested that all have 
been derived from a single homogeneous magma. But the existence of such a 
magma, even in a single province, seems unaccountable, and granting its possibility 
we do not find the suggested causes of segregation to be satisfactory. They are 
without exception either contrary to physical principles or quite incapable of pro- 
ducing the effects ascribed to them, except on a small scale. 
We are accordingly forced to consider whether there may not be other explana- 
tions of the facts. And indeed we constantly find, even in the case of limited 
exposures of the upper zones of plutonic masses, evidences of the derivation of 
material from the surrounding rocks (chiefly crystalline schists). But it is no use 
appealing merely to a certain amount of absorption as the cause of rock-variation. 
We must go back to the origin of the magmas themselves. It seems certain that 
the plutonic masses, as we know them, must result from the melting down of other 
pre-existing rocks, whether igneous, sedimentary, or metamorphic. They have 
certainly been liquid at one time or another since the earth reached its present 
stage of cooling, so that, unless we reject the solid-earth theory, they must have 
