552 GEOLOGY AND MiyEBALOGY. 



SCIENTIFIC AND LITEEAET NOTES. 



GEOLOGY AND MINERALOGY, 



ORIGIN OF ROCK CLEAVAGE. 



Few subjects connected with the physics of Geology have attracted of late years 

 more attention than th :t of rock cleavage. Long considered, in accordance with 

 the views of Sedgwick, as the result of a peculiar crystalizing force produced 

 by electrical action or by heat, its origin ha3 more recently been attributed, and 

 evidently with truth, to the effects of mechanical causes. In other words, cleav- 

 age in rocks may be regarded as the result of enormous or long-continued pres- 

 sure, exerted at right angles to the direction of the cleavage planes. AmoDgst 

 those who have chiefly labored in support of this latter view, the late Pres- 

 ident of the Geological Society of London, Daniel Sharpe, with Mr. Sorby, and 

 Professor Tyndall, may be especially cited. Observations of great interest on 

 this subject will be found in some of the recent numbers of the Philosophical 

 Magazine. 



MEAN DENSITY OF THE EARTH. 



According to the computations of the Astronomer Royal, based on his Lite pen- 

 dulum experiments at the Harton Coal Pit, South Shields, the mean density of the 

 Earth is equal to 6.566. This value is about one degree higher than any previ- 

 ously obtained. 



The Rev. Samuel Haughton of Trinity College, Dublin, in a paper communi- 

 cated to the Philosophical Magazine for July, 1856, has deduced from these ex- 

 periments, by another mode of calculation, the value 5.480. 



The officers engaged on the Trigonometrical Survey of the L'nited Kingdom, 

 have also ti.ken up the question of the Earth's density. Observations on the de- 

 flection of the plumb-line at Arthur's seat, Edinburgh, conducted by Colonel 

 James, R. E., and re-calculated by Captain A. R. Clarke (proceedings of the Royal 

 Society, May 8, 1856,) give for the Earth's mean density, the value 5.316. A fur- 

 ther set of observations on the Stack Mountain, Sutherlandshire, pointed out by 

 the late Dr. Macculloch as the best adapted in all Scotland, for the estimation 

 of the Earth's density by the deflection of the plumb-line, are also promi 



We have collected the above, and other earlier results, into the following table : 

 A. Estimated by Plumb-line Deviation. 



1. From Dr. Maskelyne's observations on the Schehallien Mountain in 



Perthshire, (corrected by Hutton) 4.9999 



2. From Colonel James' Observations on Arthur's Seat 5 316 



(The first calculations gave 5.14.) 



B. Estimated by the Ball Apparatus. 



3. By Cavendish (corrected by Baily) 5.448 



4. By Cavendish (corrected by Schmidt) 550 



5. By Reich, in Freiberg (18.37) 5.44 



6. By Baily ("mean result of over 2,000 observations) 5.67 



