114 L. W. Falcher — On the Hirnant Limestone. 



The process of formation of the Landscape Mai'ble seems to me to 

 have been mainly mechanical, although, as might be expected, there 

 is evidence also of chemical change. In attributing the corrugated 

 surfaces to the shrinking of the calcareous mud, I may have appealed 

 too strongly to mechanical causes, as apart from the obscure processes 

 of segregation, or even of concretionary action. 



The facts, however, show the connexion between the arborescent 

 markings and the corrugated and lenticular masses of banded lime- 

 stone ; and they suppt)rt the contention that the markings were 

 produced by changes amid the variously tinted calcareous mud 

 durina: its solidification. 



III. — Note on the Composition and Structure of the Hirnant 



Limestone. 



By L. W. FuLCHER, B.Sc, F.G.S. 



(PLATE IV.) 



DURING the summer of 1890, Professor Cole and I, whilst 

 geologising in North Wales, were led to examine the Hirnant 

 Limestone. Since then the further examination in the laboratory 

 of the specimens collected by myself have revealed some remarkable 

 cliaracteristics which, as they are as yet undescribed, I think are 

 worthy of notice. 



The specimens on which the following observations have been 

 made were obtained from a small cutting opposite to the farm 

 named Cwm-yr-aethnen, in the valley of the Hirnant, which descends 

 towards Bala Lake. The limestone occurs here between two beds 

 of fossiliferous slate. The exposure is very small, and being con- 

 siderably overgrown, it was only by careful search that we suc- 

 ceeded in finding it. The Survey Memoir ^ mentions that an outcrop 

 occurs at Trum-y-gwraged;i, aid also that loose blocks were found 

 by Sedgwick at the Bwlch-y-groes ; but after a careful search we 

 failed to find it at either of these localities. 



It is necessary for me to add here that I am greatly indebted to 

 Prof. Cole for much valuable advice and criticism of the facts 

 observed as well as for the loan of some material, and I hereby 

 tender him my best thanks. 



The rock is described in the Survey Memoir as " fossiliferous, 

 black and pisolitic, the concretions being about the size of small 

 grains of barley." 



It is a very tough rock, and in hand specimens from the above- 

 mentioned localit}^ the black grains, which are mostly ellipsoidal in 

 shape, are from 1 mm. to 3 mm. in their longest diameter, and are 

 somewhat sparsely scattered in a crystalline matrix, that is to say, 

 they are not so crowded together as in the case of an ordinary piece 

 of oolite limestone. The centres of the grains often show the bright 

 cleavage lace of a calcite crystal. 



An analysis of the rock reveals the interesting fact that the black- 



1 Eamsay, Geology of North "Wales, Mem. Geol. Survey, vol. iii. 



