330 Geology of 



lapilli, were deposited anew. The tunnel section in this respect is- 

 also very instructive. Thus, in course o£ time, the great crater wall 

 was formed, rising to an altitude of nearly two thousand feet, and 

 having a diameter of more than five miles at its crest. It is clear that 

 close to the vent, from which scoriae and ashes were thrown out in 

 large quantities, the greatest thickness of the agglomerate beds ought 

 to he formed, and this, in fact, is the case, as the largest beds, having 

 sometimes a thickness of several hundred feet, are situated within the 

 inner side of the caldera wall. The lava-streams here between these 

 agglomerates are irregular in their direction, and mostly of small dimen- 

 sions. The more we advance towards the outer slopes of the caldera 

 wall, the less frequent become these agglomeratic or tufaceous layers, 

 whilst the lava-streams, which towards the centre have the greatest 

 bulk, and are very stony and compact, become now gradually 

 more and more numerous, but of smaller size and more porphy- 

 ritic or scoriaceous, according to the laws by which the flow, 

 dimensions and cooling of the lava-streams are regulated. It is, 

 moreover, evident that many of them, owing to want of material 

 scarcely reach half way down the slopes of the caldera wall, that others 

 rapidly thin out, and that many which for some distance, after flowing 

 over the lip of the crater, had been of large dimensions and stony, 

 become, long before its outer edge is reached, thin and scoriaceous, so 

 that here streams of five feet in thickness are not uncommon. 

 Although the tunnel does not offer us the necessary data to judge of 

 the breadth of the lava-streams, we have for that purpose ample 

 evidence in Grodley Heads, the sea wall near Sumner, and many 

 other localities. There are streams which are 500 feet broad, others 

 only 30 to -kO, but all without exception are somewhat scoriaceous on 

 the bottom, where the lava flowing over cold ground cooled more 

 rapidly. In many instances this is well exhibited by the existence of a 

 small bed of laterite, a brick -red coloured rock, sometimes only a few 

 inches thick, which doubtless was a layer of soil on the decomposed 

 upper portion of a lava-stream, or agglomerate bed exposed for a 

 considerable time to atmospheric action before the new eruption 

 took place. The lava in the larger streams and in its central portion,. 

 principally very stony and of a blackish colour, gradually becomes, as we 

 approach the surface, more porphyritic, with a more open texture, and 

 assumes pinkish or lilac tints, till it changes into scoria?. The decom- 

 position or alteration is here often so great that it is impossible to 

 trace the top of the line of contact between the surface of the stream 

 and the bottom of the overlying: bed, both forming a layer of coarse 



