TRANSACTIONS OF THE SECTIONS. 71 



a group •would be made out'-betwcen the Labrador series, or Upper Laurentian, and 

 the Cambrian. lie considered that the attempt to identify the subdivisions of the 

 pre-Cambrian rocks in distant countries (l?ritain and iVmerica for instance) was pre- 

 mature. Calling attention to the two unconformable gi-oups which Dr. ilicks had 

 made out at St. David's, he felt satisfied that the Cambrian was unconformable to 

 the upper as well as to the lower, and stated that he had himself found fragments of 

 the horustones, i. e. the Upper pre-Cambrian group, in the conglomerates at tlio 

 base of the Cambrian. The oldest rocks of N. W. tScotland, of t!ie Malvern Hills, 

 and of Scandinavia, he thought could at present onlj^ be safely called pre-Cambrian. 



(jth Epoch. Gap between Iluronian and Cambrian. — Since we have in Britain 

 certainly two, and in America probably three series of deposits before the Cam- 

 brian, and the Cambrian may rest on any one of them, it is impossible to estimate 

 the diu'ation of the period between the Cambrian and the newest of the pre-Cam- 

 brian rocks. 



Ith Epoch. Cambrian. — He referred especially to the labours of Dr. Tlicks, and 

 thought that there were no hard and fast lines of demarcation between diflerent 

 subdivisions of the Lower and Middle Cambrian, but only zones of life, and that 

 the boundary-lines between the portions of the series in which these zones of life 

 occurred were continually being shifted. Sometimes, where a change in the sedi- 

 ment happened to come between two zones, this was seized upon as marking a con- 

 venient place to draw a line. Such a boundary was that offered by the Garth frrit, 

 which comes between the zone of Am/eUna Seclf/wickii and that of JEJt/lina binodusa. 

 No life zone older than this last appears to have been yet made out in the Lake- 

 district. This grit is not a conglomerate formed of fragments of the underlying 

 rock, but is made up almost entirely of quartz-pebbles, small and well worn, as if 

 derived from a distance. A precisely similar grit occurs associated with some- 

 what similar slate low down in the gi-een slates and porphyry in Chapel-le-Dale ou 

 the S.E. border of the Lake-district, probably not very far above the horizon of the 

 Garth Grit. It is like the grit which occm-s frequently in South Wales in the 

 Caradoc beds, in the Denbigh Grits in North Wales, and in the Lake-district in 

 the Coniston Grits, and in all these cases is known to be far above the base in a 

 conformable series. 



A great part of the series above this horizon is, in the Lake-district and in North 

 Wales, made up of volcanic ejectamenta. In North ^^'ales the ash and lava seem to 

 have been deposited in the sea and modified by its action ; while in the intervals 

 between the periods of volcanic activity various forms of marine life lived on the 

 muddy bottom, which enable us to correlate the beds with the Bala series. In the 

 Jjake-district the sea seems to have been filled up by the immense quantity of 

 material thrown out, and much of the accumulation is supposed to have been sub- 

 aerial. In both districts volcanic activity seems to have ceased, while the fauna of 

 tlie ]5ala Liniestone still inhabited the area ; and subsidence went on while the 

 Bala and Hirnant Limestones, with a great mass of interbedded and overlying 

 Hags, were deposited in North Wales ; and in the Lake-district the con-esponding 

 deposits, viz. the Coniston Limestone, Fairy-Gill Shales and Ash-Gill Flao-s 

 (= Lower Coniston Flags), were formed. In South Wales and the western borders 

 of England only a few ash-like beds suggest the not distant line of volcanic outbursts. 

 Scotljfnd, Scandinavia, Bohemia, and America yield a series which, if not in detailj 

 can in a general way be correlated with these. The fact that the Lake-district 

 and North Wales were during this period the seat of old volcanoes, will partly explain 

 the difficulty that was experienced by Prof. Sedgwick and Sir Roderick Murchison 

 in identifying the corresponding beds in the two areas independently examined by 

 them ; and the sudden ending of the volcanic deposits may probably account for the 

 local apparent irregularity of the Coniston or Bala Limestone on the underlj-ing 

 series, which induced Professor Sedgwick to make that limestone the base of his 

 upper subdivision, and which has recently been urged as proofs of an unconformity 

 by Mr. .Eveline. Except in connexion with the volcanic deposits, no break has 

 been proved from the conglomerates which form the base of the Harlech group to 

 the top of the Bala series. 



Sth Epoch. Tlie Gap between the Catnbrian and Sihtrian. — This he thought not 

 Strongly marked, and certainly not to be drawn between the Upper and^Lower 



