Ivi REPORT — 1861. 



the present, are the result of agencies still at work. But whilst it is his 

 high office to record the distribution of life in past ages and the evidence of 

 physical changes in the arrangement of land and water, his results hitherto 

 have indicated no traces of its beginning, nor have they afforded evidence of 

 the time of its future duration. Geology has been indebted for this progress 

 very largely to the investigations of Sedgwick and the writings of Sir 

 Charles Lyell. 



As an example of the application of geology to the'^practical uses of life, 

 I may cite the discovery of the gold-fields of Australia, which might long 

 have remained hidden, but for the researches of Sir Roderick Murchison in 

 the Ural Mountains on the geological position of the strata from which the 

 Russian gold is obtained. From this investigation he was led by inductive 

 reasoning to believe that gold would be found in similar rocks, specimens of 

 which had been sent him from Australia. The last years of the active life 

 of this distinguished geologist have been devoted to the re-examination of 

 the rocks of his native Highlands of Scotland. Applying to them those 

 principles of classification which he long since established, he has demon- 

 strated that the crystalline limestone and quartz-rocks which are associated 

 with mica schists, &c., belong by their imbedded organic remains to the 

 Lower Silurian rocks. Descending from this well-marked horizon, he 

 shows the existence beneath all such fossiliferous strata of vast masses of 

 sandstone and conglomerate ol' Cambrian age ; and, lastly, he has proved the 

 existence of a fundamental gneiss, on which all the other rocks repose, and 

 which, occupying the North-western Hebrides and the west coasts of Suther- 

 land and Ross, is the oldest rock-formation on the British Isles, it being 

 unknown in England, Wales, or Ireland. 



It is well known that the temperature increases, as we descend through 

 the earth's crust, from a certain point near the surface, at which the tem- 

 perature is constant. In various mines, borings, and artesian wells, the 

 temperature has been found to increase about 1^ Fahr. for every 60 or 65 

 feet of descent. In some carefully conducted experiments during the sinking 

 of Dukinfield Deep Mine (one of the deepest pits in this country), it was 

 found that a mean increase of about 1° in 71 feet occurred. If we take the 

 ratio thus indicated, and assume it to extend to much greater depths, we 

 should reach at two and a half miles from the surface-strata at the tempera- 

 ture of boiling water ; and at depths of about fifty or sixty miles the tem- 

 perature would be sufficient to melt, under the ordinary pressure of the 

 atmosphere, the hardest rocks. Reasoning from these facts, it would appear 

 that the mass of the globe, at no great depth, must be in a fluid state. But 

 this deduction requires to be modified by other considerations, namely, the 

 influence of pressure on the fusing-point, and the relative conductivity of 

 the rocks which form the earth's crust. To solve these questions a series of 

 important experiments were instituted by Mr. Hopkins, in the prosecution 

 of which Dr. Joule and myself took part ; and after a long and laborious 

 investigation, it was found that the temperature of fluidity increased about 

 1° Fahr. for every 500 lbs. pressure, in the case of spermaceti, bees-wax, and 

 other similar substances. However, on extending these experiments to less 

 compressible substances, such as tin and barytes, a similar increase was not 

 observed. But these series of experiments has been unavoidably interrupted ; 

 nor is the series on the conductivity of rocks entirely finished. Until they 

 have been completed by Mr. Hopkins, we can only make a partial use of 

 them in forming an opinion of the thickness of the earth's solid crust. 

 Judging, however, alone from the greater conductivity of the igneous rocks, 

 we may calculate that the thickness cannot possibly be less than nearly three 



