314 C. N. FENNER 



the surface of the ground this structure is not so evident. This is 

 mostly due, no doubt, to the leaching-out of the secondary minerals, 

 but the impression is left that in its original occurrence it may not 

 have been so well developed in the upper layers. 



The course of events which gave rise to these features is to be 

 explained as follows: Before the lava flow occurred this site was 

 occupied by a shallow body of water with a muddy bottom. When 

 the first thin flow of intensely heated igneous rock plunged into it, 

 the violent agitation resulted in a thorough mixture of the two. At 

 short intervals further flows added to the depth of the mass. The 

 successive spurts and tongues of fused material burst forth everywhere. 

 Chilled almost at once by the steam pouring around them, they built 

 up a structure of bowlder-like forms. The original body of water 

 was quickly driven off, but that contained in the saturated strata 

 beneath was changed to steam and rushed up through the mass. 

 Some of it found its way into and through the fused material, but the 

 greater part worked its way around and among the bowlders, with the 

 result of quickly cooling the crusts, producing a glassy texture and 

 a multitude of cracks. The evidence of these effects throughout 

 the seventy or eighty feet to the surface indicates that at least this 

 depth of lava had reached its position while the action was still con- 

 tinuing, though it does not follow that there may not have been inter- 

 vals during which the flow of lava ceased. 



Further than the physical effects of chilling the lava and pro- 

 ducing the crusts and cracks, the steam seems to have had little result. 

 In places the inner surfaces of the blow-holes have no hning of miner- 

 als but appear entirely unaltered. In another locality, which will 

 be described later, many of the steam vents were blown full of sand 

 or dust and this frequently lies up against the chilled glass, without 

 any indication of alteration in the latter. Later, after a considerable 

 interval of time, the mass of lava cooled to such a degree that the 

 underlying reservoirs began to supply a mixture of water with the 

 steam, and finally merely heated waters passed up through the vents, 

 and with these the crystallizing action and formation of minerals 

 began. The lava had been so thoroughly seamed and cracked that 

 it could not have offered much obstruction to the flow of these waters, 

 and it does not seem that thev could have been at anv time under a 



