338 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



"often carried off several leagues from the volcano. This fine 

 " dust is termed volcanic ash, and furnishes the principal material 

 " for the formation of the layers of tuff which are so abundant in 

 " volcanic districts. The fragments of lava possess at the moment 

 "of their ejection from the crater very different temperatures. 

 " Some of them, especially at the commencement of the eruption, 

 " are scarcely warm, and possess the dark colour of scoriae ; others 

 " in greater quantity are red and white hot — the latter remain for 

 " a short time fluid and perfectly plastic, form themselves into 

 " rotating ellipsoids, or adopt some times abnormal long drawn 

 li forms. These latter singular pieces have been termed volcanic 

 ** bombs. Decreasing in size, and becoming mixed with small 

 " angular fragments, they graduate into what has been called by 

 " the Italians, Lapilli, or volcanic sand." * 



When the eruption has reached its climax, and the whole of 

 the crater to a certain level has become filled with lava, the latter 

 breaks out from beneath the dark crust that generally overlies it, 

 at the lowest point of the bank of the crater, and rolls down the sides 

 of the volcano, forming what appears as a stream of fire by night, 

 and a thick viscid stream of slag by day. The lava leaves the 

 crater red hot and as fluid as melted metal, but shortly afterwards 

 the stream cools and becomes solid on the surface, while it remains 

 for a long time fluid in the inside, the heat there hidden showing 

 itself here and there through the cracks in the solidified crust. 

 As the stream rolls on, these cracks close up, while others form at 

 other places. " The whole surface is in continual motion ; at 

 " one point large bubbles are observed swelling up, which finally 

 " burst and leave their rugged sides behind standing erect in 

 " the most curious forms ; at another point cakes of slag in the 

 " most varied positions are carried along ploughing furrows as 

 " they go, or tearing half fluid lava with them and drawing it out 

 " and winding it round in curious rope like forms (the so called rope 

 " lava). At some points the surface folds itself into deep cylin- 

 " drical canals, which run on beside each other and parallel with 

 " the direction of the stream ; and at others, crossfolds and depres- 

 " sions are formed. Thus these lava streams present, in that part of 

 " their course where this struggle between their fluid interior and 

 " the solidified crust has taken place, an extraordinarily wild and 

 " rugged appearance." f 



* Die vulcanishe Gesteine in Sicilien nnd Island, p. 155. 

 f Naumann, Lehrbuch, I, 161. 



