IxXXvi PBOCEEBDfGS OF THE GEOLOGICAL SOCIETY* 



which, whilst the whole excavation was drowned, would naturally 

 descend to its deepest parts. 



To resume. A number of anomalies and irregularities obtrude 

 themselves among the effects of the terrestrial temperature which, 

 although they throw no sort of doubt upon the doctrine of its pro- 

 gressive increase, by no means tally with the deductions of theory 

 and physical experiments. Our lamented former President, Mr. 

 Hopkins, established, by direct experiment, the fact of certain rock- 

 substances, such as the denser limestones, granites, and syenites, 

 having a conducting-power (for heat) of twice, thrice, or four times 

 that possessed by the less dense materials, chalk, clay, and sand- 

 stone, and that the conduction of heat is very sensibly affected, 

 although not to any great amount, by discontinuity in the conduct- 

 ing mass. Prom these data theory would infer that, if the con- 

 ductive power of a certain rock be double that of another, the in- 

 crease of depth corresponding to a given increase of heat would in 

 the former case be double of what it is in the latter. Hence in fact 

 Hopkins himself admitted that, the conductive power of the strata 

 of the Paris basin being only about half that of the Coal-measures, 

 the rate of increase of temperature in the Artesian well of GreneUe 

 ought, according to his theory, to be nearly t^ice that of the pit of 

 Dukiniield in Cheshire, whereas, from the observations made at 

 that time, down to the depth of 1330 feet, the disparity in the two 

 cases appeared to be very slight. The further prosecution of the 

 shaft, however, gave a nearer approximation to the theoretical result, 

 in showing 76-8 feet to 1° Pahr. as against 60 feet at Crenelle*. 



It cannot but be admitted that, however much the observations 

 made by the small cohort of accurate observers may show varying 

 rates, their imiformity is more remarkable than their divergence, 

 and this with a gTeat disregard to the nature of the masses, which, 

 as regards their quality, are shown by experiment to possess very 

 different degrees of conductive power. iLr. Hopkins, in order to 

 explain the anomaly, tests the problem of a deep isothermal surface 

 being in a position not parallel with the exterior of the globe, but 

 allows that there are no conceivable grounds for the admissibility of 

 this very limited hypothesis according to the theory of central heat. 

 But, on the other hand, Cordier showed, in 1827, that the rate of 

 augmentation of temperature in the same class of rocks (the Coal- 

 measures) of neighbouring departments of Prance is in one case 

 double, in another nearly treble that of a third ; and from these 

 apparently imperfect data he inferred that the subterranean heat is 

 distributed with much irregularity in different districts. On review- 



^ A remarkably slow rate of progression is shown, as I am informed by Mr. 

 W. J. Henwood, P.E.S., at the celebrated gold-mine of Morro Yelho, in Brazil, 

 situate at a height of 3250 feet above the sea, and opened in clay-slate rock. 

 The water issuing from the rock at 45 fathoms depth, observed in 1843, had a 

 temperatm-e of 69°, that at the bottom of the mine in 1863 and 1864, at 145 

 and 155 fathoms deep, 72°. These temperatures were quite independent of the 

 effects of the warm rains a little before and after Christmas, which make them- 

 selves felt all the way down the engine-shafts. The rate of increase would 

 hence be but one degree for 200 feet. 



