78 Reviews — Delesse's Lithologie du Fond des Merst 



are very numerous in volcanic regions ; besides, their outpour be- 

 comes very abimdant when eruptions or earthquakes take place. 



The formation of mountain-chains has also given rise to extra- 

 ordinary activity in mineral springs ; moreover, when rocks have 

 been raised from the bottom of the sea and brought back above its 

 surface, the waters which saturated them must have flowed under- 

 ground, giving rise to powerful infiltrations. 



However it may be, the substances incessantly introduced into the 

 sea by mineral waters have subsequently been separated by organized 

 beings, and also by chemical precipitation. Beds of carbonate of 

 magnesia or dolomite have been doubtless formed in the latter way, 

 both at the present time and in former epochs. Such is also the 

 most probable origin of gypsum, which may be attributed to the 

 freeing of sulphuretted hydrogen, or else of sulphated mineral waters; 

 in most cases, besides, gypsum appears to have been deposited in 

 lakes. 



"II. — Eruptions. 



Eruptions, those in particular which occur at the bottom of the- 

 sea, contribute in a very efificient manner to the formation of 

 marine deposits, and even give rise to strata ; for they accumulate a 

 great quantity of debris, which are afterwards re-assorted by waves 

 and tides, and then carried off by currents. 



The geological study of rocks shows well all the importance of 

 these eruptions, and often even marine beds are almost entirely 

 composed of debris of rocks which have been thrown up from the 

 interior of the earth. This is particularly observable in trappean, 

 basaltic, and trachytic tuffs, and for all tuffs,, in short, which are 

 formed from volcanic rocks. 



At the present day submarine eruptions are rather rare, and, 

 besides, they only take place at a small number of points ; they 

 must, however, be taken into account in studying the lithology of 

 our sea-bottoms. They give, indeed, deposits resulting from volcanic 

 ejectamenta, and therefore entirely distinct from those which are 

 habitually formed. These deposits grouj) themselves in the neigh- 

 bourhood of the voleanos which produce them ; Avhen, however, the 

 volcanic ejectamenta are in the state of floating pumice, of minute 

 debris, of ash or of mud, they may easily be transported by currents, 

 and then be spread to very great distances. 



As regards France, we know no submarine voleanos near our 

 coasts, either in the ocean or the Mediterranean ; but in the latter 

 sea, the islands of Julia and Santorin enable one to judge of the 

 importance of the effects occasioned by recent submarine eruj)tions. 



Now there ai-e also salses or mud-volcanos which specially eject 

 gases, water, and mud, and which in consequence give rise to beds 

 of clay. Such voleanos are found either at the surface of the land 

 or at the sea-bottom. In the Caspian, more particularly, their 

 ejectamenta have sometimes formed shoals, and even islands of mud, 

 rising to the surface of the water. 



The mud which is being deposited at the present time in our lakes 

 and at the bottom of our seas nearly always contains carbonate of 



