Geological Society. 231 



solving the equation, it can be determined whether the real roots 

 and the real parts of the imaginary roots be negative. 



When the question has been treated thus far, a number of sub- 

 sidiary points come under notice : e. g. in the case in which the 

 system has a force-function, the equation f (m)=0 contains only 

 even powers of m. This case also presents several points for discus- 

 sion, such as the difference between oscillations about a position of 

 equilibrium, and about a position of steady motion. Amongst other 

 points which come up for discussion we will instance one more, to 

 which a separate chapter is devoted, viz. the question under what 

 circumstances it is necessary to examine the terms of the second 

 order in order to assure ourselves of the stability of the motion ; 

 for it is possible that some of these may have their periods so timed 

 that their effects accumulate until the character of the motion is 

 changed. 



It is well known that, though it was intended to be given every 

 alternate year, the Adams prize is but rarely awarded. We believe 

 that on the occasion of Mr. Eouth's Essay the award was made for 

 only the fifth time since the year 1848. This is a fact which 

 renders any praise of ours superfluous. 



XXXIV . Proceedings of Learned Societies. 

 GEOLOGICAL SOCIETY. 



December 5, 1877.— Prof. P. Martin Duncan, M.B., F.R.S., 



President, in the Chair. 



[Continued from p. 158.] 



rpHE following communications were read : — 

 ■*• 3. " On some Precambrian (Dimetian and Pebidian) Eocks 

 in Caernarvonshire." By Henry Hicks, Esq., F.G.S. 



In this paper the author gave an account of the special examina- 

 tion of the great ribs of so-called intrusive felspathic and quartz 

 porphyries which are found associated with the Cambrian rocks in 

 Caernarvonshire, made by him in company with Prof. Hughes, Mr. 

 Hudleston, and Mr. Homfray last summer. He described sections 

 at and near Moel Tryfan and across the mass from Pen-y-groes to 

 Talysarn, in which he showed that instead of being of an intrusive 

 nature, as hitherto supposed, the whole, with the exception of a few 

 dykes at those parts, is made up of bedded volcanic rocks, lavas, 

 breccias, &c, similar to those found in the Pebidian series at St. 

 David's, and that the Cambrian rocks, instead of being intruded by 

 this mass, rest everywhere upon it unconformably, and the pebbles 

 in the conglomerate of the Cambrian at the base are, as at St. 

 David's, identical with, and must have been derived from the rocks 

 below. Similar results were obtained in the examination to the 

 north and south of Llyn Padarn ; and the conclusion, therefore, at 

 which the author has arrived with regard to the great mass which 



