Pseudo-Morphous Crystals of Quartz. li9 



tiiis difficulty will, I think, be obviated by the consideration that 

 the points of contact may be, for the most part, in very narrow 

 fissures, or mere cracks in the rocks, and that the water, being 

 greatly heated, may be much less than four times the density of 

 the steam in immediate contact with it. A continual struggle 

 would, no doubt, exist between the water and steam under such cir- 

 cumstances, so that, in many places, they would alternately encroach 

 beyond the line of demarcation ; but as the checks on both would 

 increase in proportion to the extent of their encroachments from 

 the diminution of the temperature above, and its augmentation 

 below, such encroachments would probably not be very extensive 

 or of long duration under ordinary circumstances. Suppose a tem- 

 porary encroachment of the water on the limits of the steam to 

 occur at one point ; the steam would probably encroach on the 

 water at another at the same time, and then reactions taking place, 

 the effects would be reversed. Thus assuming, what indeed would 

 appear to follow from admitted data as necessary consequences ; 

 steam would not only exist below the water, but such oscillations 

 would tend to give motion and activity to the water in the neigh- 

 bouring fissures, causing it to circulate in the earth more or less 

 freely and extensively according to circumstances. In volcanic dis- 

 tricts, where the heat may be great at comparatively small depths, 

 analogous phenomena sometimes occur at the surface, which are 

 probably caused by the action and re-action of steam and water. 

 Amongst these may be included the intermitting Geyser springs in 

 Iceland, as well as some of the mud volcanoes found in Sicily, and in 

 Asia and America. 



It seems probable that earthquakes may be produced by the ac- 

 tion of highly elastic vapour, rapidly generated at great depths, in 

 consequence, perhaps, of copious and sudden influxes of water into 

 intens-ely heated parts of the earth ; and their lines of direction 

 jire doubtless influenced by those of the fissures or veins of the dis- 

 tricts in which they occur. But these are phenomena of compara- 

 tively rare occurrence, and it is no wonder that they should be so, 

 when we consider how vastly greater must be the force required to 

 uplift the rocky crust of the earth and wrench it asunder, than that 

 which will support a column of water equal to the thickness of that 

 crust. 



Since the foregoing paper was read, I have rather hastily exa- 

 mined some other portions of water taken from different pseudo- 

 morphous crystals. One of those portions contained muriatic and 

 sulphuric acids, iron, a trace of lime, and of common salt. Acid was 

 a little in excess, and some peroxide of iron was left in the cavity 

 from which the water was taken. In another, the same acids were 

 detected, and some iron. In the thii-d portion, there seenied to be 

 nothing besides a little common salt. In many of the octohedral 



