Reports & Proceedings — Geological Society of London. 93 



He describes the results of some high-pressure mimetic experiments, 

 aided by a Royal Society »rant which he now gratefully acknowledges. 

 These experiments were designed to produce this structure, and reveal 

 what the author believes to be many new points on the origin of 

 concretions and cone-in-cone in particular. The experiments are new, 

 inasmuch as the media used, namely, brittle, semi-plastic, and plastic, 

 are enclosed in tunics of varied design, and then subjected either to 

 a high uniform hydrostatic pressure or to a direct thrust. The 

 results are in many ways analogous to those of Ewing, Goodman, and 

 Daubree, who, it is remarked, did not attempt to explain cone-in-cone 

 structures. 



The author concludes from the evidence — 



(1) That cone-in-cone is not due to crystallization, but is a mechanically 



produced structure due to great and localized pressure. 



(2) That it is closely allied to the phenomenon known as pressure solution. 



(3) That cone-in-cone structure is closely associated with other rock- 



structures which are mutually indicative the one of the other, and 

 also of their mode of origin. 



3. January 20, 1915.— Dr. A. Smith Woodward, F.R.S., President, in 



the Chair. 



1. " The Geology of the District around Machynlleth and Llyfnant 

 Valley." By Professor Owen Thomas Jones, M.A., D.Sc, F.G.S., 

 and William John Pugh, B.A.., of the University College of Wales, 

 Aberystwyth. 



In an introduction a brief account is given of the physical features, 

 general succession, and structure of tbe area, and reference is made to 

 the work of previous investigators, especially Walter Keeping. For 

 the major group the classification applied in 1909 to the district 

 around Plynlimmon and Pont Erwyd is adopted, but slight differences 

 are introduced in the arrangement of the minor groups. The 

 classification is as follows: — 



' a TArp J- Pale mudstones with numerous laminated grit-bands. 



Valentian 

 (Silurian). 



Stage. / 



B. Pont 

 Erwyd { 

 Stage. 



Derwen 

 Group. 

 190 feet. 



Cwmere 

 Group. 

 334 feet. 



r 5. Zone of Mcmograjptus " 



halli. Pale- blue and 



4. Zone of Monograptus greenish mud- 



sedgwicki. stones, with 



3. Zone of Monograptus bands of dark 



regularis. \ graptoliferous 



2. Zone of Monograptus shales, and 



leptotheca. some thin 



1. Zone of Mesograptus green flags. 



magnus. J 



("3. Monograptus spp. ^ Dark rusty 



beds. \_ 



Zone of Diplograptus I 



acuminatus. J 



Zone of Glyptograp- 



tus persculptus. 



Haetfell 

 (Ordo- 

 vician). 



A. Plyn- 

 limmon 

 Stage. 



weathering 

 shales and 

 limestones. 

 Mottled Beds 

 and blue 

 mudstones. 



Ty'n-y-maen f Dark mudstones, grits, and some 

 Group. I conglomerates. 



