398 Geological Society :- — 



If M f =0, then the motions in 6 and <£ will be independent of each 

 other. If M is also 0, then we have the relation 



...... LPQ+NRS = 0; . . . . . . (13) 



and if this is fulfilled, the disturbances of the motion in 6 will have no 

 effect on the motioning. The teeth of the differential system in 

 gear with the main shaft and the governor respectively will then 

 correspond to the centres of percussion and rotation of a simple 

 body, and this relation will be mutual. 



In such differential systems a constant force, II, sufficient to keep 

 the governor in a proper state of efficiency, is applied to the axis 

 77, and the motion of this axis is made to work a valve or a break on 

 the main shaft of the machine. 3 in this case is merely the fric- 

 tion about the axis of £. If the moments of inertia of the differ- 

 ent parts of the system are so arranged that M' = 0, then the dis- 

 turbance produced by a blow or a jerk on the machine will act in- 

 stantaneously on the valve, but will not communicate any impulse to 

 the governor. 



GEOLOGICAL SOCIETY. 



[Continued from p. 317.] 



March 11th, 1868.— Prof. T. H. Huxley, LL.D., F.R.S., President, 

 in the Chair. 



The following communication was read ; — 



" On the Structure of the Crag-beds of Norfolk and Suffolk, 

 with some observations on their Organic remains. — Part I. Coral- 

 line Crag." By Joseph Prestwich, Esq., F.R.S., F.G.S., &c. 



The history of the division of the several crag-deposits into three 

 formations — the Mammaliferous, Red, and Coralline Crags — having 

 been recounted, the author stated that for the last thirty years the 

 evidence of their sequence had remained unaltered, the distinction 

 between the Mammaliferous and Red Crags being still purely 

 palseontological, not a single case of superposition having been dis- 

 covered. Mr. Prestwich then proceeded to the special object of 

 this paper, which was to describe more fully the physical structure 

 of the several crags, and to determine, if possible, the exact relation 

 which the Suffolk Crags bear to the Crag of Norfolk. 



Commencing with the Coralline Crag, the author stated that the 

 well-known outlier at Sutton furnishes a base-line and the best clue 

 to its structure and dimensions, showing also the depth to which it 

 has been denuded and replaced by the Red Crag. The Coralline 

 Crag is generally described as consisting of two divisions — an upper 

 one, formed chiefly of the remains of Bryozoa, and a lower one of 

 light-coloured sands with a profusion of shells; and the author 

 now gave their exact dimensions and his proposed subdivisions, as 

 follows : — 



