^04. Philip Lake — Bala Lake and River Sydem. 



Fig. 5. — Portion of the auterior groove as seen on. the inside of the test, drawn'i 

 from a wax squeeze of the internal cast ; x 5 diaui. ia. iuteranihuhicrals. 

 p. pores for the passage of the podia or of the ampulla} from the podia. 

 f.p. flooring-plates. The proximal end is uppermost iu the drawing. 



Fig. 6. — The internal cast of part of a subvective groove ; the complement of 

 Fig. 5. X 5 diam. p'. projecting scars of matrix that filled the podial' 

 pores. 



Fig. 7. — The distal end of the left posterior groove, from a Avax squeeze of the- 

 external impression ; x 4 diam. Lettering as in Fig. 5. 



Fig. 8. — The central part of the abactinal surface, from a wax squeeze of the- 

 external cast (PL IX), slightly diagi-ammatised, since details are often obsciu-e 

 in the specimen ; x 3 diam. per. peripheral area. fr. frame, cent, central 

 area, with its flexible plated membrane, raised in five interradially situate 

 lobes. * position of posterior (anal) interradius. p.ia. the meridian in which 

 plates of the posterior interradius (i.e. posterior interambulacrals) pass from 

 the actinal to the abactinal surface and merge with the peripheral plates of 

 the latter. 



II. — Bala Lake and the Kivee. System of North Wales. 



By Philip Lake, M.A., F.G.S. 



Introduction. 



THE evolution of rivers and the origin of lakes have recently 

 attracted a considerable share of attention ; and the principles 

 which should guide us in tracing the development of a drainage 

 system have been expounded by several authors, both in England 

 and in America. 



Little, however, has been written on the lakes and rivers of 

 North Wales. In 1876 Ramsay,^ who was one of the pioneers in 

 questions of this kind, endeavoured to decipher the history of the 

 Dee and of Bala Lake ; and subsequently he performed a similar 

 office for the Clwyd.- More recently Mr. Strahan and Mr. Morton^ 

 have dealt with the latter river. 



A few papers,* also, have been written on the tarns or ' Llyniau ' 

 •of Caernarvonshire ; but with these we are not directly concerned. 



In the following pages an attempt is made to explain the origin 

 of Bala Lake, and to show how the river system of North Wales 

 was established and was subsequently modified ; but the formation 

 of the Vale of Clwyd is barely touched in this investigation. 



It is scarcely necessary to point out how deeply I am indebted 

 to the writings of Mr. Marr, to which, in fact, whatever there may 

 be of value in my labours is primarily due. For although the 

 present drainage system of North Wales differs greatly from that 

 of Cumberland and Westmoreland, yet iu some respects the resem- 

 blance is so close that it is Mr. Marr's work in the Lake District 

 which has enabled me to attempt an interpretation of the river 



1 Quart. Journ. Geol. Soc, vol. xxxii (1876), pp. 219-229. See also Mem. Geol, 

 ■Surv., vol. iii (1881), 2nd ed., pp. 314-323. 



2 Mem. Geol. Surv., vol. iii (1881), 2nd ed., pp. 306-313. 



3 G. H. Morton: Proc. Liverpool Geol. Soc, vol. viii (1897-98), pp. 32-65, 

 181-204. A. Strahan: Geol. Mag., 1899, pp. 111-117. 



* W. W. Watts : Brit. Assoc. Eeports, 1895, pp. 683, 684. W. A. Brend : 

 Geol. Mag., 1897, pp. 404-407. J. E. Marr & 11. H. Adie: Geol. Mag., 1898, 

 pp. 51-61. 



