402 J. Starlcie Gardner — Underground Heat. 



come across them. Yery valuable information as to the depths at 

 which high temperatures are seated could be gathered in non-volcanic 

 regions where there are hot springs by observing the outcrop of the 

 underlying impervious stratum and estimating the depth at which it 

 would be situated where the springs issue forth. Thus the rainfall on 

 the Mendips may probably feed the thermal springs at 120° F. at 

 Bath, and the depth from which the water rises would not be more 

 than the thickness of the strata forming the basin — about 4200 feet. 

 Again, the fact that springs of scalding water are met with in some 

 mines of 2000 feet in depth, as at Redruth and the Comstock, makes 

 it unlikely that the horizon of boiling water is anything like so low 

 as the 10,000 feet estimated from the observed increment in artesian 

 wells. We can hardly in fact conceive that the hot springs so pre- 

 valent in Germany and other parts of Europe, away from both 

 volcanic districts and earthquake zones, rise from such prodigious 

 depths ; though they may of course do so. In volcanic districts at 

 least, there can however be no question as to the proximity of under- 

 ground heat to the surface, for its presence is apparent in fumaroles, 

 geysers, mud caldrons, etc., not always close to vents, but often in 

 plains at some little distance. 



But the great proof of the thinness of the earth's crust is to be sought 

 in the movements which are constantly taking place at its surface. 

 Its extreme sensitiveness has only become apparent since the micro- 

 phone has enabled earth-tremors to be appreciated, and Ave now know 

 that the solid ground is sensitive to a footstep and vibrates under a 

 hail-stone. The seals the fixed, and terra firma the unstable element. 

 Every accumulation of weight brought from elsewhere by any trans- 

 porting agency causes the land to sink in a corresponding degree. 

 The whole story of the sedimentary rocks is one continued record of 

 subsidence keeping pace with sedimentation. Every layer of the 

 Coal-measures was formed at or very near the sea-level, while the 

 crust was giving way inch by inch under the continued additions of 

 weight, until in South Wales it finally sagged no less than 10,000 

 feet. The entire Cambrian series appears to have been formed in 

 uniformly shallow water, and yet in our area it forms a mass esti- 

 mated at 23,000 feet in vertical thickness, or sufficient to have filled 

 in solid the abyssal depths of the Atlantic. Every river delta that 

 has been pierced shows an endless succession of estuarine beds, often 

 hundreds of feet in thickness, but each of which is recognized by its 

 contents to have been formed at the water's level. No sea-coast-line 

 is at rest. Where cliffs are being eroded and carried away by the 

 waves, there is upheaval ; for it will be noticed that, irrespective 

 of the general strike inland, the strata of which they are com- 

 posed almost invariably dip away from the sea on the shore-line. 

 This general dip can be traced all round even deep and land-locked 

 bays, and is explicable on no other hypothesis. Where sediment is 

 accumulating out at sea, great subsidence must take place ; for were 

 it otherwise, coasts would be surrounded with vast shoals but a few 

 feet below the influence of the surf, and extending as far as ever the 

 land had stretched before the sea began to act upon it. Islands 

 separated off by sea-action generally exhibit a strike for the strata 



