m 



"With reference to inequalities arising from the friction of a resist- 

 ing medium at the earth's surface, the author observes that they 

 could not exist, if, as in the manner here shown, the axis of rota- 

 tion coincided from the origin with the axis of figure. 



In conclusion, he remarks, that if we could assume for the planets 

 a similarity of physical constitution to that of the earth, the theorem 

 as to the difference of the greatest and least moments of inertia of 

 the earth would be applicable to all the planets ; and thus w^e should 

 be as well assured of the stability of our system, with respect to the 

 motion of rotation of its several members, as we are already respect- 

 ing their motion of translation. 



In a postscript, referring to a third cause of disturbance in the place 

 of the earth's axis of rotation, suggested in a letter from Sir John 

 Lubboclc, namely, the effects of local elevation and depressions at 

 the earth's surface, the author states ; if, with Humboldt, we re- 

 gard the numbers expressing the mean heights of the several conti- 

 nents as indicators of the plutonic forces by which they have been 

 upheaved, we shall readily see that these forces are of an inferior 

 order to those affecting the general forms and structure of the earth. 

 If the second class of forces acted so as not to influence in any way 

 the stability of the earth's axis of rotation, the former class might, 

 under certain conditions, produce a sensible change in the position 

 of the axis. But when the tendency of the second class of forces is 

 to increase the stability of the earth's axis, it w^ould not be easy to 

 show the possibility of such conditions as to render the operation of 

 the other forces, not only effective in counteracting that tendency, 

 but also capable of producing a sensible change in the place of the 

 axis of rotation. 



3. A paper was in part read, entitled, "On the Arrangement of 

 the Foliation and Cleavage of the Rocks of the North of Scotland." 

 By Daniel Sharpe, Esq., F.R.S., V.P.G.S. Received November 20, 

 1851. 



February 19, 1852, 



WILLIAM SPENCE, Esq., V.P., in the Chair. 



The reading of Mr. Sharpe's paper, " On the Arrangement of the 

 Foliation and Cleavage of the Rocks of the North of Scotland," was 

 resumed and concluded. 



The author applies the term, cleavage or lamination, to the divi- 

 sional planes by which stratified rocks are split into parallel sheets, 

 independently of the stratification ;/o/z«^20/?, to the division of crystal- 

 line rocks into layers of different mineral substances ; slate, to strati- 

 fied rocks intersected by cleavage ; and schist, to foliated rocks only 

 which exhibit no bedding independent of the foliation. 



He considers that no distinct line can be drawn between gneiss 

 and mica schist, chlorite schist, &c., which pass from one into the 



