J. Stnrlde Gardner — Underground Heat. 399 



we get access to it, and will it prove more tractable ? These will 

 develope into the burning questions of the future. 



We are aware of the presence of this central heat in various ways, 

 the most direct of which is the observed increase in the temperature 

 when the crust is bored into. A great deal of evidence upon this 

 has been collected by the committee appointed by the British Asso- 

 ciation to investigate underground temperatures, and the result of 

 their work was to ascertain that the increase varies from 1° F. for 

 from every 130 to every 34 feet. Several still newer observations 

 give a result near the latter figure. The mean is about 1° in 

 64 feet, the rate greatly depending on the conductivity of the 

 matrix bored. It would perhaps be safer to take a rather higher 

 average, as it seems likely that borings, whether filled with air or 

 water, must as a general rule give a rather lower temperature than 

 the rock itself. It also seems probable that the rate may increase 

 in a compound ratio at depths beyond those hitherto reached, since 

 radiation would be less. The observed increase would give us boil- 

 ing water at 10,000 feet, and molten rock of the temperature of lava 

 as it issues from Vesuvius, 2000° F., at a depth of 20 miles. This 

 is only calculated, however, from what we know to take place within 

 the first three-quarters of a mile from the surface, the greatest 

 depth yet tested, and we shall see in the sequel that there are other 

 reasons for anticipating that these temperatures will be met with 

 nearer the surface. 



There are, as is well known, whole classes of rocks called meta- 

 morphic, that is to say, rocks originally sedimentary, which have 

 become crystalline through heat. They comprehend the gneisses, 

 schists, slates, and nearly all crystalline rocks indeed which are not 

 igneous or intrusive. There is even reason to suppose that granite 

 itself is a clastic rock which has been fused at a great depth from 

 the surface. They have been proved to be merely sedimentary rocks, 

 originally formed as ordinary sea or other mud, by tracing the same 

 strata from their crystalline to their normal and often fossiliferous 

 condition. In the latter case we can be quite sure of their age, and 

 are able to estimate the greatest possible depth at which they are 

 ever likely to have been buried, by the simple process of adding 

 together the maxima of thickness of all the more recent known 

 sedimentary strata. Some have been metamorphosed by direct con- 

 tact with igneous rocks, but the greatest work has been effected 

 through heat generated by squeezing and pressure during the 

 elevation of mountain chains. These are plicated rocks, but all 

 metamorphic rocks are not plicated, and since the microscopic in- 

 vestigations to which they have been subjected leave no doubt as 

 to heat having caused the alteration, we may suppose in such cases 

 that at the depth under which they were buried the temperature 

 must have been sufficient to cause metamorphosis. In basing any 

 theory upon them, however, we must proceed with extreme caution, 

 for we must first be quite sure that the heat was not the result of 

 squeezing and movement ; and, secondly, we cannot be sure of the 

 depth of sediment which existed above them, though we have fair 



