276 JR. M. Deeley — Mountain Building. 



The trilobation of the thorax is nearly lost in ^. Knighti (which 

 is paralleled by H. planus, Sandb., otherwise quite distinct), and the 

 axis of the pygidium is scarcely marked off from the lateral lobes, 

 the axial furrows in both cases being almost obsolete. The elongated 

 triangular shape of the pygidium, its numerous annulations, and its 

 produced and pointed extremity are features which are common also 

 to Trimerus, Digonus, and Burmeisteria, sens, str., as noticed below. 



H. Johannis differs from H. Knighti in possessing a broad pre- 

 glabellar area, which results in the head-shield having a triangular 

 appearance instead of being transverse and so much shortened as to 

 be broader than long. In consequence of the length of the head 

 being not thus abnormally reduced, the convergent facial sutures 

 approach each other in front more closely before bending abruptly 

 inwards to form the transverse commissure. We shall observe a 

 similar modification in members of Digonus and Burmeisteria. 



The hypostome of H. Knighti has been figured by Lindstrom,^ and 

 is of the same type as that of H. ( Trimerus) delphinocephalus. 



{To be continued.) 



V. — Mountain Building. 

 By E. M. Deeley, M.Inst. C.E., F.G.S. 



IN an article by Dr. H. Jeffreys in the Geological Magazine for 

 April, pp. 215-19, an attempt is made to show that the discovery 

 of radio-active materials in the earth's crust favours the " contraction 

 and puckez'in'g " theory of mountain building. With regard to 

 0. Fisher's view he writes, "It rests entirely on Kelvin's theory of 

 the cooling of the earth, which has had to be completely revised on 

 account of the discovery of the extensive distribution of radio-active 

 matter in the earth's crust. The time available has been found to 

 be about twenty times greater than on Kelvin's theory, and the 

 cooling has therefore had time to extend to a much greater depth 

 and to produce a very much greater compression." We are also 

 told what the reduction in the earth's diameter has probably been, 

 and the actual depth of Fisher's level of no strain. 



Reference is also made to the three very valuable and interesting 

 papers hj Arthur Holmes published in the Geological Magazine. 

 But Holmes is by no means as dogmatic as Dr. Jeffreys. Holmes 

 writes : " If each grain of the earth's substance were as rich in radio- 

 elements as are the rocks which have been examined, the earth's 

 total output of heat from this source alone would, in any given 

 period, be about 300 times as great as the amount actually lost by 

 conduction to the surface and radiation into space." 



" This astonishing result pulls us up sharply, for it is manifestly 

 absurd to believe that our planet is becoming hotter at the appalling 

 rate implied in these figures, or, indeed, that it is becoming hotter at 

 all." To get over the difficulty it is suggested that the radio-active 

 elements only exist in the outer portion of the crust of the earth, and 

 in quantities insufficient to cause the earth to become hotter, and 



^ Lindstrom, Handl. k. Svensk. vet. Akad., Bd. xxxiv, No. 8, p. 57, t. iv, 

 figs. 20, 21, 1901. 



