382 Correspondence—Ir. T. Mellard Reade. 
Part I. —The character of the limestones which underlie the plat- 
form of Vesuvius may be studied in the peninsula of Sorrento, where 
the mass attains a thickness of 4700 feet. They are magnesian in 
varying proportions. A table was given showing twenty-seven 
analyses, made principally by Ricciardi, the amount of MgO ranging 
from 1 to 22 per cent. Silica rarely exceeds 2 or 38 per cent., where- 
as in the greater number of limestones it isabsent. The bituminous 
matter, though a powerful colouring agent, usually exists in quantities 
too small for estimation, but sometimes reaches 8 per cent. Such 
are the materials out of which the extraordinary series of silicate-, 
compounds have been developed, and as these materials of themselves 
could not form peridotes, micas, pyroxenes, etc., it is clear that the 
silica, alumina, iron, fluorine, etc., must have been introduced from 
without, viz. from the neighbouring igneous magma. The author 
then discussed the question of the probable methods, being inclined 
to favour the notion of vapour in combination with acid gases. 
The bulk of the paper was occupied with a detailed description of 
the microscopic structure of these stratified limestones and their 
derivatives. The author remarked that the same metamorphic 
changes may be traced on a much grander scale amongst the ejected 
blocks, and hinted at the similarity of these changes to those of 
contact-phenomena as seen elsewhere, and even of regional meta- 
morphism, the two main factors to be considered being the composi- 
tion of the rock to be acted upon and that of the magma acting. 
The changes which ensue in an impure limestone are, in the first 
place, the carbonization of the bituminous contents, which are con- 
verted into graphite; and a kind of recrystallization, approaching 
the saccharoidal structure, seems to have taken place, although the - 
stratification, etc., is preserved. A few grains of peridote now begin 
to make their appearance, chiefly as inclusions within the calcite 
crystals, and thus by degrees the results already recorded are effected. 
In the early stages only is the metamorphism selective. The order 
in which the new minerals seem to develope is the following :—(1) 
Peridote, Periclase, Humite. (2) Spinel, Mica, Fluorite, Galena, 
Pyrites, Wollastonite. _(8) Garnet, Idocrase, Nepheline, Sodalite, 
Felspar. Many of these minerals are crowded with microliths, 
which there is reason to believe consist of pyroxene. 
CORRESPONDENCE. 
a 
ELEVATION AND SUBSIDENCE. 
Str,—In the suggestion as to the cause of subsidence and eleva- 
tion put forward by Professor Lloyd Morgan,’ it is not quite clear 
whether on his hypothesis he looks upon the conversion of molten 
rock into the crystalline condition as a case of simple condensation 
by pressure following ordinary lavas, or whether he assumes that 
after a certain pressure is applied the molten rock will suddenly 
assume the crystalline condition and contract, and thereby cause 
1 Grou. Maa. July, 1888, pp. 291-97. 
