io8 GEOLOGY 



a hard plutonic rock, and the lithographic limestone of Solenhofen, typical of a soft 

 sedimentary deposit. The samples were worked into the form of short columns, care- 

 fully perforated axially and transversely, and were embedded in accurately fitting jackets 

 of steel: intense pressure was then slowly applied by means of pistons. The period of 

 pressure varied from a few hours to between two and three months, and the temperature 

 ranged up to 55oC. Any contraction of either the longitudinal or the transverse boring 

 was ascertained by means of an accurately gauged wire fitting into the perforation. 

 These experiments lead to the conclusion that empty cavities may exist in granite at a 

 depth of at least eleven miles, which is a greater depth than previous estimates had sug- 

 gested. 



In connection with Prof. Adams's researches the subject has been discussed mathe- 

 matically by Mr. Louis V. King, who concludes that, so far as hydrostatic pressure in 

 the earth's crust is concerned, a small cavity at normal temperature will remain open at 

 depths down to between 17.2 and 20.9 miles. 



These researches have more than academic interest. When Sir C. A. Parsons 

 (inventor of the steam turbine) proposed some years ago that an experimental bore- 

 shaft, for geological and mining exploration, should be sunk to a depth of perhaps 12 

 miles, it was objected that such a depth was unattainable since the walls would collapse 

 by the viscous flow of the rocks. This objection is now quite untenable. Prof. Adams's 

 investigations have an obvious bearing on the question of the depth to which mineral- 

 veins and other ore-bodies may descend. Whether formed by deposition in fissures 

 or by replacement of rock, it would seem that free spaces are required for reception of 

 mineral matter. It is now seen that, in so far as fractures control deposition, ^re- 

 deposits may well continue in depth to at least n miles, which is probably a greater 

 depth than could ever be reached by mining operations. 



The Planetesimal Hypothesis. To the Journal of Geology (Vol. xix, 1911, p. 673) 

 Prof. T. C. Chamberlin, of Chicago, has contributed a paper in which he shows how his 

 planetesimal hypothesis has been strengthened by the discovery of radioactivity. 

 According to his view the earth originated in a nuclear knot of a spiral nebula, around 

 which it was slowly built up by the aggregation of planetesimals, or discrete masses 

 revolving in elliptical orbits around the common centre of the system. By the crossing 

 of their paths, the planetesimals were gradually gathered into the immature earth. A 

 certain amount of heat would be derived from the condensation of the original nucleus, 

 and more would be developed by the infall of the captured planetesimals, but the chief 

 source of the internal heat of the earth was found, according to the author, in the com- 

 pression which the growing earth would suffer as the deeper parts were pressed by the 

 rock-masses that gradually accumulated above. When this hypothesis was originally 

 enunciated doubt was not unnaturally expressed as to the adequacy of progressive self- 

 compression to account for the earth's internal heat, but any doubt as to a sufficient 

 supply of heat was dispelled on the discovery of an additional source of thermal energy 

 in radioactivity. 



As the earth was formed by the aggregation of all kinds of planetesimals derived 

 from the original nebula, it must have presented from the beginning a very heterogene- 

 ous composition. On sufficient elevation of temperature, the less refractory substances 

 would pass into a state of fusion, or rather mutual solution, but such liquefaction was 

 essentially selective, local and temporary. At no period of the earth's history was 

 there general fluidity, like that of a molten globe. It is held that the local liquid 

 matter would be squeezed outwards by stress-differences in the body of the earth, 

 forming threads or tongues which would flux their way upwards along paths of easy re- 

 sistance, and thus transfer heat from the central parts towards the surface. 



It seems a fair assumption that originally radioactive matter would be distributed 

 through the heterogeneous mass of planetesimals quite promiscuously. But as the 

 presence of any heat-generating bodies would favour the liquefaction of neighbouring 

 matter, this matter would tend to rise towards the surface of the growing earth, carrying 

 with it the associated radioactive substances. According, then, to the planetesimal 



