276 Prof. J. D. Dana on some Results of 



which exhibits a peculiar relation between the time-intervals i[ 

 and i 2 and the mean values of the ergal and vis viva. 



If the mass /a be taken as very great in comparison with m, 



so that the fraction — — may be supposed equal to m, the pre- 

 ceding equation changes into that which holds for the motion of 

 one material point about a fixed centre. This equation I derived 

 separately in a memoir published a short time since*; and I 

 then said that for two points moving about each other the cor- 

 responding equation could in like manner be derived. Here, 

 however, the same equation has come out as a special case of a 

 much more general one. 



To equation (II.) various other forms can be given, which are 

 both theoretically interesting and convenient for use, wherein it 

 can at the same time be brought into connexion with my theorem 

 of the virial. These transformations and especially the applica- 

 tion of the equation to the theory of heat I reserve for a subse- 

 quent memoir. 



XXXIII. On some Results of the Earth's Contraction from Cool- 

 ing, including a discussion of the Origin of Mountains. By 

 James D. Dana. 



[Continued from p. 219.] 

 IV. Igneous Ejections, Volcanoes^'. 



THE direct connexion of igneous ejections with the move- 

 ments resulting from the earth's contraction has been 

 briefly illustrated in the course of the remarks on mountain- 

 making J. It is apparent, without further discussion, that 

 regions of great disturbances embrace the regions of great ig- 

 neous ejections — that the oceanic slopes of the border moun- 

 tains, especially around the Pacific, the greater ocean, have been 

 preeminently subject to such eruptions — and that in Tertiary 

 times, when the earth's crust was becoming too stiff to bend 

 much before the lateral pressure, profound fractures instead of 

 flexures were a common result of its action, igneous outflows 

 were most extensive, and volcanoes were of increased size and 

 numbers. 



The questions remaining are : — 



(1) Does the mobile rock owe its mobility in all cases, or any, 

 to the movements of the rocks under lateral pressure — motion 



* NacJir. der K. Gesellsch. der Wiss. zu Gottingen, Dec 25, 18/2; 

 Phil. Mag. July 1873. 

 t For Part I. see p.41 ; Part II. p. 210; Part III. p. 217. 

 X S^r«, pp. 52, 137, 212, 214. 



