226 Proceedings of Royal Society of Edinburgh. [sess< 
(Note added April 22, 1904.) 
The remainder of the paper dealt with theoretical considerations, which 
may be summarised as follows : — 
In explanation of the facts, it is suggested that four chief factors are con- 
cerned, which are as follows : — (1) Earth movements, which generate the heat 
required for volcanic action, and also furnish the motive power by which the 
magma is forced outwards from the focus. (2) The presence, at the focus of a 
volcano, of saline waters, whose dissolved salts become concentrated by pro- 
longed boiling, and the consequent escape of steam at the surface. These 
saline solutions, operating at high pressures and temperatures, dissolve the 
rock in various directions around the volcanic focus, and add their own alkalis 
to the magma so formed. (3) An excess of alkalis (especially of soda) in the 
magma, whereby it is enabled to gradually extend its ramifications into the 
rock around its focus. (4) Circulatory movements from the extremities of the 
system to the volcanic focus and back, analogous to the movements of the hot 
water in the pipes of a heating apparatus. This circulation behaved in a 
manner analogous to that of the circulatory system in a tree, in which the 
leaves generate one set of products, and the roots carry in another, in the 
shape of water and alkalis. These commingle, and then travel outwards from 
below, to be finally left in the solid form, and thus contribute to the extension 
of the whole. 
An ordinary sedimentary aggregate, to which the dissolved constituents of 
sea-water had been added, operating under high temperatures and pressures, 
might furnish the materials of the basic and sub- basic eruptive rocks ; while 
the granitic materials constituting the floor of the Earth’s crust could supply 
the additional potash and silica required for the formation of acid and sub-acid 
series of rocks. 
It was further suggested that many basalts, and most gabbros, were of 
secondary origin, and that their present structure is due to changes which have 
originated within the core of a volcano. Some basaltic tuffs had thus been 
softened and reconsolidated as pseudo-massive rocks ; while many basalt lavas, 
dykes and sills, occurring within the same zone of reconstruction, appear, in 
like manner, to have been softened and then recrystallised into gabbro. Most 
granophyric granites associated with gabbros may represent such changes 
carried further still, and may be due to the solvent action of a granite magma 
upon an older set of basic rocks (see fig. 27 above). 
The bearing of these considerations upon various other metamorphic pro- 
cesses connected with the origin of gneisses and rocks allied thereto, was 
discussed in some detail. 
{Issued separately , May 20, 1904.) 
