BASAL CAMBKIAN ROCKS OF SHROPSHIRE. 419 



of some of them with the actual volcanic debris. Again, if we go 

 further afield, in North Wales, as I have shown *, the rocks which 

 overlie the Monian unconformably, and are so well developed between 

 Bangor and Carnarvon, are very volcanic in character, yet they form 

 the base of the Cambrian and have several well-marked conglomerates 

 in their midst, up to the one that overlies the felsites of Llyn Padarn. 

 It may be that the Uriconian rocks are of the same age as these, — 

 they have both been thought to be the same as the Pebidian. And 

 this takes us to St. Davids. There is certainly there an entire 

 unconformity and independence between the Cambrian conglomerates 

 and the underlying series, and therefore I so unhesitatingly classed 

 the volcanic group of St. Davids with the Middle Monian, that I 

 gave as an alternative name for that subdivision " the St. Davids 

 group."' Eut there is no special similarity between the rocks there 

 and tlie Middle Monian of Anglesey ; there is quite as much resem- 

 blance, if not more, to the rocks between Bangoi- and Carnarvon, 

 It is possible, therefore, that Dr. Geikio may be right after all, and 

 that sufficient allowance has not been made, in estimating the signi- 

 ticance of the unconformity, for the volcanic nature of the underlying 

 rocks. Again, in the ^lidlands, the Nuneaton tiuart/ite lies upon 

 the volcanic rocks of Caldecott : and at Charnwood the Swithland 

 slates have great conglomerates associated with them and pass down, 

 without any discovered break, into the volcanic ashes and lavas of 

 the western portion of the Porest. If, on further knowledge, it 

 should turn out that all these must bo referred to the Cambrian, it 

 would be certainly a remarkable circumstance, amounting to a 

 generalization, that wheresoever we reach the base of that formation 

 we find it formed of volcanic debris, which is not, perhaps, an 

 unnatural commencement of a system formed under new physical 

 conditions. 



And if this should turn out to be the case, which as yet is far 

 from proved, then would the Monian system bo entirely cleared 

 from the Pebidian and its associated Arvonian and Dimetian, and 

 stand firmly as a distinct system founded entirelj' on independent 

 observations. 



EXPLANATION OF THE MAP, PLATE XVI. 



The Monian and Cambriau Eocks of Shropshire. 



This Map is traced from the 1-inch Ordnance Survey Map ; the geological 

 lines being reduced from the 6-inch Survey Map. Consequently, if this 

 Map be retraced on the Ordnance Map, the exact positions of all lines will 

 be found. 



Discussion. 



Dr. Hicks said it was greatly to ])e regretted that Dr. Callaway, 

 who had done so much in unravelling the geology of the district 

 referred to was unable to be present to reidy to the Author's 

 remarks. In his absence, he (Dr. Hicks) desired to say that, in the 



* Q. J. O.S. vol. xliii. 



