Reviews—New Survey Maps and Memoirs. 175 
acid and bases are set free. These combine with more magma, and 
form acid and basic silicates. Further addition of water then sets up 
new decompositions, but some water always remains free in the 
magma, although on account of its relatively small amount it has 
a very low vapour-pressure, in accordance with: Raoult’s law. The 
process can go on till a great part of the magma is decomposed. The 
magma must also increase in volume on taking up water, by very 
nearly the volume of the water absorbed. 
The swollen magma, which has also become more fluid, exerts 
pressure on all sides, and the column rises still further in the vent 
which is in connection with the magma-basin. On rising, the magma 
is cooled in the narrow vent, the water continuously becomes a 
weaker acid, and is displaced by silicic acid from the hydrates; tlfe 
pressure of the aqueous vapour rises in spite of the fall of 
temperature, and if the watery layer comes near enough to the 
surface, and therefore under sufficiently low external pressure, steam 
explosions will occur. 
According to this theory a volcano acts very lke a geyser. As the 
water rises in a geyser, so it does in a volcano, though in this case it 
is mostly in chemical combination with the magma. At great depths, 
and therefore under high pressures, the water in the geyser and the 
watery magma in the volcanic vent are under higher pressures than 
those corresponding to the maximum tension of the water-vapour, and 
no explosion is possible. When a magma enters the vent of the 
volcano and rises in it, it at last reaches a point where, on account of 
separation of water from the magma through cooling, the pressure of 
the water-vapour exceeds the external pressure. Then an explosion 
occurs, blowing a passage through the overlying layers, and thus 
leading to renewed explosions by relief of pressure. ‘This goes on till 
so much water has separated from the magma that the remainder cannot 
overcome the external pressure. ‘Then ensues a period of temporary 
quiescence, with slow loss of water-vapour by diffusion, until after 
a sufficient lapse of time so much water penetrates into the magma- 
basin that the process begins anew. 
Bey ea Ve WV SS = 
——_—_ 
I.—New Grotocicat Survey Mars anp Memorrs, 
1. Tur GeoLtoey or FarmourH anp Truro and oF tHE Minine District 
or CamBorNE AND Reprure. By J. B. Hirt, R.N., and D. A. 
MacAuisrer, A.R.S.M., F.G.8.; with Petrological Notes by 
J. 8. Frerr, M.B., D.Sc. 8vo, cloth; pp. x and 336, 24 plates. 
Pages 1-112 form Part I, Geology; pp. 1138-314 form Part IT, 
Mining; pp. 315-324 are Bibliography ; and pp. 325-335, Index. 
Price 7s. 6d.; in explanation of colour-printed map, Sheet 352, 
ls. 6d. (EK. Stanford: London, 1906.) 
2. Tur GroLocy oF THE Country NEAR Neweavay. By Crement Re, 
F.R.S., F.L.S., F.G.S., and J. B. Scrtrvenor, M.A., F.G.S.; with 
contributions by J. 8. Frerr, M.A., D.Sc., W. Potzarp, M.A., 
