CAMBRIAN IX CAERXAEVOXSEIEE. 249 



other connexions, and. in the complicated districts, other faults, might 

 possihly be as consistent with the same necessaril}' imperfect data 

 as those that are drawn. My own is only one possible solution, 

 but the Geological Survey map is in many cases an impossible 

 one. These doubts, however, affect only the surface-distribution. 

 Of the vertical succession there can be no doubt whatever. 



§ 3. The Pale Slates. 



These, which are also called Green Slates, have attracted atten- 

 tion as lying at the top of the series in the great quarries of 

 Penrhyn and Llanberis, and are of particular interest in the former, 

 as having yielded the only recognizable fossil in the series. I do 

 not think, however, that their colour has any special geological 

 significance. As Prof. Hughes remarks, they are not cut off by a 

 sharp line from the Purple Slates, and as far as I can judge they are 

 merely a discoloured portion of the latter. For in the Llanberis 

 quarries we see similar slates in the neighbourhood of the green- 

 stone dykes, and when these latter do not reach the surfac3 the 

 paleness of the slate is often a guide to where they occur towards 

 the bottom of the workings. In accordance with this we find bands 

 of pale slate distributed sporadically amongst the purple, as at 

 Llanllechid and Nan tile. Their occurrence in certain places at the 

 top, though in greater quantities than usual elsewhere, may there- 

 fore be merely an accident, due possibly to their exposure to 

 weathering before the deposition of the Bronllwyd Grit. 



§ 4. The Purple Slates axd the St. Axx's Grit. 



The great mass of this slate is divisible into several portions, 

 which have been dwelt upon by Sir A. Ramsay, and more recently 

 have been described by Prof. Hughes {op. cit.). In particular there 

 is a band of reddish grit about the middle and a thin band of 

 green grit nearer the bottom, but still above the St. Ann's Grit. 

 These, however, are too impersistent or too easily misplaced to be 

 appreciable on a map, and for geological purposes the whole must 

 be taken together. It is known to be much broken, contorted, 

 and faulted, but these accidents have only an economical importance, 

 and need not be further noticed. 



The St. Ann's Grit is a more important mass which, when present, 

 is not easily missed, and thus it serves as a guide to the strati- 

 graphy. It receives its name from its occurrence on the hill on 

 which St. Ann's Church, Bethesda, stands. It is sometimes very 

 fine-grained and compact, and sometimes coarser ; in the latter case 

 it contains small quartz-pebbles which, as noted by Prof. Hughes, 

 in certain localities are rosy, but as a whole the grit is character- 

 istically green. At St. Ann's it is thrown into a sharp anticlinal 

 fold whose axis runs N.N.E., and in the centre of which the Lower 

 Purple Slates come up in a long tongue, while to the north-west 

 the corresponding broader svnclinal on the hill-slopes below Sling 



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