164 Notices of Memoirs — D. Forhes, F.E.S., 



altered condition, belonging to the Laurentian series of Canada, and 

 as yet geologists have not been able to discover what these sedi- 

 mentary beds may in their turn rest upon, i.e., what is actually 

 beneath them. 



Since we therefore have not as yet been able to reach down to 

 examine directly any rocks lower in the geological series than those 

 pertaining to the Laurentian formation, we will now turn to the 

 Volcanos, in order to examine the mineral products which they bring 

 up for our consideration, from depths vastly lower than those which 

 we can ever hope to reach directly. What Volcanos teach us with 

 regard to the nature of the Earth's interior at the depth from which 

 they derive their supply of molten mineral matter, may be summarized 

 as follows : — That at this depth, the earth's substance exists in a 

 state of molten liquidity, forming as it were a sea of melted rock or 

 lava, analogous in character to the eruptive rocks which have in 

 former ages broken through the earth's crust ; secondly, that the 

 mineral products ejected by volcanos are very similar in chemical 

 and mineral constitution, no matter from what part of the globe 

 they may emanate ; and lastly, that from the same A'-olcanic orifice, 

 and during the same eruption, lavas of two totally different classes 

 may be emitted, viz. the light acid or trachytic lava, analogous to 

 the granites, felsites, etc., of the oldest periods, and the heavy basic 

 pyroxenic lava, all but identical with the dark basaltic or trappean 

 rocks commonly met with, as dykes, etc., intersecting and disturbing 

 most of the different sedimentary formations. 



Besides these, another deduction from the study of volcanic 

 phenomena, indicating that at a certain depth below the surface the 

 volcanic vents must be in connexion with a continuous sea of 

 molten lava, is based upon the influence which the moon appears to 

 have on volcanic eruptions, an opinion which seems to have been 

 confirmed by the observations by Professor Palmieri made during 

 the last outburst of Vesuvius, on which occasion he reported that 

 distinct tidal phenomena could be recognized, thereby indicating 

 that the moon's attraction occasioned tides in the internal zone of 

 molten lava similar to those it causes in the ocean. A further 

 corroboration of this view is seen in the results of an examination of 

 the records of some 7000 earthquake shocks which occurred in the 

 first half of the present century, compiled by Perry, and which, ac- 

 cording to him, demonstrate that earthquakes are more frequent in 

 the conjunction and opposition of the moon than at other times ; 

 more so when the moon is near the earth than when it is distant, and 

 also more frequent in the hour of its passage through the meridian. 



Eeturning now to the more direct examination of the superficial 

 parts of the earth, we find that the results of mining operations 

 have also thrown considerable light, not only on the mineral nature 

 of the rocks encountered in depth, but also upon some of their 

 physical conditions. A numerous set of experiments made in deep 

 mines in various parts of the world, far distant from one another, 

 have most conclusively proved that the temperature of the earth, at 

 least as deep down from the surface as has yet been explored by 



