400 J. Starkie Gardner. — Underground Heat. 



grounds for believing that even the Silurian system has not been 

 buried under more than 25,000 feet of newer strata, 



A more striking manifestation of the existence of heat beneath us 

 is seen in volcanic eruptions. We learn by them that rocks are not 

 only incandescent, but rendered molten and fluid by the intensity of 

 the heat. It is now recognized that the explosions, showers of stones 

 and ash, and escape of volumes of steam, which accompany eruptions 

 and render them so terrible, are caused by the sudden conversion of 

 underground waters in more or less superficial strata into steam. 

 The great absorption of water that takes place in volcanic regions 

 has often been remarked, and so soon as the heated lava is set in 

 motion and commences to rise, it is certain that it must come into 

 contact with saturated strata and underground waters, with the 

 inevitable result that most violent explosions must occur. The 

 longer the interval between two eruptions, the more deeply and 

 completely the rocks to be rent will be saturated, and the more 

 energetic the explosion ; and eruptions are accordingly observed to 

 be violent in proportion to the period of quiescence which preceded 

 them. The explosions and steam are the mere accidental accom- 

 paniments of an eruption, which is fundamentally the squeezing out 

 or escape through a vent or crack of some of the molten interior, 

 owing presumably to an increase of pressure elsewhere. After the 

 paroxysmal stage, that is, when the water in contact has all been 

 blown into steam, the lava flows tranquilly enough, and may con- 

 tinue to well out for very long periods. It- seems probable that 

 eruptions through craters are very minor matters, and that the grand 

 eruptions are through fissures which may extend for hundreds or 

 even thousands of miles in almost parallel lines. The Eocene 

 basalts of Ireland and Scotland are generally agreed by geologists to 

 have formed part of an extensive tract that once stretched uninter- 

 ruptedly through the Faroes to Iceland and Greenland. These lavas 

 are in horizontal sheets, and there is no trace of any craters through 

 which they could have welled, but the fissures are plain and marked 

 by large dykes. Their outpouring seems to have been unaccom- 

 panied by explosions, for nowhere have I discovered any great 

 deposits of scoriae or lapilli interstratified with them, and contem- 

 porary deposits that have formed in their vicinity show no layers of 

 ash. The Chalk also is quite or almost free from volcanic dust, 

 while the corresponding ocean ooze of the present day invariably 

 contains a large percentage of such extraneous matters. The results 

 of other large and equally tranquil eruptions are to be seen in the 

 Deccan, where there is an area under lava 200,000 square miles in 

 extent and 6000 feet in thickness, and in Oregon an undulating 

 plain of basalt equalling the combined areas of France and England. 

 These facts are merely mentioned to show that volcanic eruptions 

 are not merely local phenomena, caused by lateral thrusts or pressure, 

 but are vents through which the heated and molten rocks of the 

 interior overflow the surface. When we see rocks rise and burst 

 through the crust in every part of the earth at temperatures of 

 2000° F., we can hardly refuse to believe in underground heat. 



