546 A. Harker—“ Magmatie Concentration.” 
conditions which led to the solution of spicules and the formation of 
flints was the presence of decaying organic matter; conversely it 
is not unlikely that the preservation of spicules is due to the absence 
of decaying organic matter at the time of their embedment. 
It is known that some kinds of siliceous sponges periodically shed 
their spicules, and such shed spicules might be transported some 
distance by currents. The spicules in the Lower Chalk of Wilts 
seem to have been distributed in this manner, for remains of 
sponge-mesh are rare in that Chalk. We may assume, therefore, 
that when this Chalk was accumulated the amount of decaying 
protoplasm was very small, and consequently the spicules were not 
dissolved. In the Upper Chalk, on the other hand, and especially 
in the Micraster zones, there were evidently extensive sponge-fields, 
and yet spicules are seldom found except in the interior of flints 
which are silicified sponges. Does not this suggest the inference 
that the decay of the sponges set up reactions which caused first 
the solution of the scattered spicules and ultimately the precipi- 
tation of the silica in the form of flint ? 
VI.—BerrtHetor’s PRINCIPLE APPLIED TO MaGmatic ConcENTRATION.? 
By AurrepD Harker, M.A., F.G.S. 
T is now a recognized fact that many igneous intrusions, 
especially of basic rocks, grow more basic in composition 
from the centre to the margin. This type, of what Vogt terms 
“«magmatic concentration,” has been discussed by several geologists, 
who agree in supposing that in such cases a magma, originally homo- 
geneous, has become differentiated by the migration of the less 
soluble (more basic) ingredients to the cooler marginal region of 
the magma-reservoir. 
Taking the same ground, I wish to say a few words as to the 
physical cause of such migration. The explanation which has been 
offered, and may be regarded as the current one, invokes what has 
been named ‘“Soret’s principle,” and is based on the generally 
admitted analogy between an igneous rock-magma and an ordinary 
saline solution. Soret? found by experiment that when different 
parts of a solution, originally homogeneous, were maintained for 
several weeks at different temperatures, the salt in solution tended 
to accumulate in the cooler part of the liquid. Van t’ Hoff,’ in his 
memoir on “osmotic pressure,” arrived theoretically at the same 
result, and showed that for equilibrium the concentration of the 
salt in different parts of the solution must be inversely proportioned 
to the absolute temperature. Those results of Soret which gave the 
highest degree of concentration agreed with this law, and it seems 
probable that in his other experiments sufficient time was not 
allowed for complete equilibrium to be effected. 
The principle just noticed seems at first sight to lead to an 
1 A paper read before the Brit. Assoc., Section C, at the Nottingham meeting. 
2 Annales de Chimie (5), vol. xxl. pp. 2938-297 (1881). 
3 Zeits. f. phys. Chemie, vol. i. (1887). 
