36 



Notes and Comments, 



per cent, of its original thickness ; the thickness of some clays 

 must have diminished still more ; while the amount of minute 

 cavities in rocks with slaty cleavage is so small, that sometimes 

 they are nearly solid. 



EXTENT OF PRESSURE UPON ROCKS. 



By the measurement of green spots in slates it can be 

 deduced that the rock before cleavage was somewhat more 

 consolidated than rocks of the Coal-Measures now are, and was 

 then greatly compressed and the minute cavities almost com- 

 pletely squeezed up. The development of ' slip-surfaces ' 

 in cleaved rocks is very great, and furnishes an additional proof 

 that the cleavage is of mechanical origin. ' Pressure-solution ' 

 was also dealt with. 



In conclusion, the author discussed the volume of minute 

 cavities in clay-rocks and their analogues of various ages, and 

 showed that there is a distinct relation between it and the prob- 

 able pressure to which the rocks have been exposed. Tables 

 were given of the pressures so calculated for rocks of various 

 geological ages, the volume of empty spaces decreasing in older 

 rocks from the 32 per cent, existing in recent clays. In the 

 Moffat rocks, with very little or no slaty cleavage, the pressure 

 is calculated at about 7 tons to the square inch, while the Welsh 

 slates, with ver}^ perfect cleavage, indicate a pressure of about 

 120 tons to the square inch. 



LORD KELVIN. 



By the death of Lord Kelvin, which took place at his 

 residence, Netherhall Largs, on the Ayrshire Coast, in December, 

 Britain has lost her greatest natural philosopher. He was in 

 his eighty-fourth year, and died from the effects of catching a 

 severe chill. He was born at Belfast on June 26th, 1824. At 

 22 years of age he was appointed to the Chair of Natural 

 Philosophy at the University of Glasgow, where he dis- 

 charged his professional duties for fifty-three years. He was 

 present at the meeting of the British Association at Leicester a 

 few months ago, and the ovation he received when he appeared 

 on the platform at the opening ceremony was evidence of the 

 esteem in which he was held by the members there 

 assembled. His remains were interred in Westminster Abbey, 

 and probably on no previous occasion have so many prominent 

 scientific men met within the Abbey walls as when Lord Kel- 

 vin's remains were taken to their last resting place. 



