306 ORIGIN OP ERUPTIVE AND PRIMARY ROCKS, 



increase with the depth, there must exist at some depth, sufficient- 

 ly great, a point beyond which the rocks are heated to such an 

 extent that before water can penetrate to them it is resolved into 

 steam and expelled. 



Beyond this point there is a long interval, regarding the in- 

 crease of temperature in which, we have no direct evidence until 

 we arrive at that furnished by the fused rock which in the form 

 of lava is poured forth by volcanoes, which are even more widely 

 and generally distributed over the earth's surface than thermal 

 springs. This however supplies indirect evidence sufficient to 

 prove that during this great interval the heat must in- 

 crease with the depth, until the temperature of fused lava 

 is reached, at which point we must suppose everything to 

 be in a fluid state, and consequently the temperature 

 from that point to much greater depths to continue about the 

 same. The lavas which have been emitted by volcanoes in historic 

 times, have been both of a trachytic and a basaltic nature, but 

 those of the latter character seem to have predominated. Many 

 of these doleritic or augitic lavas from very recent lava-streams 

 have been described and analysed. They are of a comparatively 

 basic composition, seldom contain more than 50 per cent of silica, 

 and are much richer than other volcanic rocks in iron-oxide. 

 The lava which constituted the stream from Etna, that destroyed 

 a great part of Catania in 1669, had the following composition: — 



Silica 48 . 83 



Alumina 16.15 



Protoxide of iron 16. 32 "J 



Protoxide of manganese 54 J 



Lime 9.31 ns nh 



Magnesia « 4.58 



Soda with some potash 3.45 



Potash 77 J 



99.95* 



This analysis bears a general resemblance to those of other 

 augitic lavas. It also bears a resemblance to that of the slag 

 produced in smelting the copper schists of Mansfeldt. According 

 to Hoffman, the composition of the slag produced at Kupfer 

 Kammerhiitte in the first or raw smelting, is as follows. 



• Jiischof ; Chemical and Physical Geology, ii, 235. 



