288 On some Results of the Earth's Contraction from Cooling. 



cause has aided in the great majority of eruptions in non-vol- 

 canic regions. 



Another cause of ejection appealed to is pressure from the 

 vapours imprisoned in the regions of fused rock. It must have 

 often given efficient aid. But such vapours may not exist to 

 the extent sometimes supposed about the deep sources of the 

 material of fissure-eruptions. Non-volcanic igneous rocks are 

 usually solid throughout, without the minutest vesicle ; and 

 similar complete compactness characterizes many fissure-ejec- 

 tions even of volcanic regions when they have taken place at a 

 considerable distance from the volcanic vent*. This would 

 hardly be so generally the fact if vapours were abundant about 

 the sources of the ejected material ; for such vapours would imply 

 the existence in the mobile rock itself ordinarily of vapori- 

 zable ingredients capable of easy vaporization under the pres- 

 sure there existing. The fact seems to be that the great pres- 

 sure is in the way of vaporization of the sulphides that may 

 exist in the plastic material ; for pyrites is often found in the 

 solid basalt. And, further, the deep-seated sources of igneous 

 rocks must be mostly or wholly below the regions accessible to 

 moisture ; for if not, they would show its presence by hydration 

 and frequently a vesicular structure. 



Volcanoes in their states of ordinary activity and eruption do 

 not appear to be dependent on the lateral pressure in the earth's 

 crust. As I have long since urged, sustaining the view of 

 Prevost, the force engaged is chiefly pressure from the expan- 

 sion of vaporizable material rising with the lava. Besides this, 

 there is the hydrostatic pressure of the liquid column raised in 

 the conduit through the expanding vapours. The want of sym- 

 pathy between the summit-crater of Mauna Loa, nearly 14,000 

 feet in elevation, and the larger crater of Kilauea on the flanks 

 of the same broad mountain only 4000 feet above the sea, I have 

 adduced as evidence that the ordinary volcanic action was here 

 due to movements in the upper parts of the lava columns, pro- 

 bably to portions extending little below the sea-level, and that 

 these volcanoes were therefore mainly dependent for their 

 various phases on the freshwaters precipitated over the moun- 

 tain-slopes. The w r aters of the ocean take their part in such 

 action ; but they are not in Hawaii the chief source of activity f. 



* It is a great, though common, mistake to suppose that volcanic ejec- 

 tions are slags or scoria. The surface of an outflow is often of this cha- 

 racter for a depth of 6 inches or perhaps a foot. But below this the layer 

 is usually a compact stony mass with nothing slag-like, except that it is 

 somewhat vesicular. Ahout Hawaii, part of the rock is as solid and free 

 from vesicles as the dolerite of the Connecticut valley. 



t See further on these points the author's 'Expl, Exp. Geol. Report/ 



