418 PKOF. J. h\ BLAKE ON THE MONIAN AND 



of Brittany, referred by French geologists to the Cambrian. This 

 band must therefore belong, by all the tests at our disposal, to a 

 system which is at once of considerable thickness and wide distri- 

 bution, and I venture to suggest that this can be none other than 

 that system A^'ho.se lower parts we see in Anglesey, in other words 

 the Monian ; and the portion of that system that is here represented 

 must certainly be an upper part. So far, then, my original sug- 

 gestion has been justified. 



But, in view of what is seen in this locality, my former suggestion 

 that the Uriconian rocks were Middle Monian cannot be adhered to. 

 If there be any Middle Monian in this district it must be the 

 Bushton schists ; and if any Lower Monian, the fragments of gneiss 

 and hornblende-schist caught up in the outburst on Primrose Hill. 

 In fact, the proof that these volcanic hills are of later date than 

 the Longmynd slates has entirely nullified all my previous con- 

 clusions as to the age and position of this and other volcanic Pre- 

 Cambrian groups, which have been too rashly assumed to belong 

 necessarily to the underlying subdivision called the Middle Monian. 



What, then, are we to do with these Uriconian rocks ? We have 

 three alternatives to choose from. Either we must class them with 

 the Monian, or we must assign them to an intermediate system, or 

 we must class them with the Cambrian. 



The uptilting of a group of rocks and the formation of fractures in 

 its base, out of Avhich a volcanic eruption may take place, indicates 

 a great change of physical conditions, and probably a lapse of time 

 equivalent to an unconformity. Can rocks formed after all this 

 still belong to the same system ? Moreover, the various conglomer- 

 ates which cap them, some forming actually part of their mass, 

 others worked up with them, show a gradual change of character 

 into those which lie upon the surface, and which resemble the Cam- 

 brian grits. These considerations seem to associate these rocks, in 

 spite of the unconforma])]e quartzites, more with the rocks above 

 than with those below. 



Again, to class them as a separate system under the title of Uri- 

 conian is little more than a confession of ignorance. Such a system 

 has been nowhere worked out, and we might have to create a new 

 system on the same principle for every localit}^ Probably, however, 

 this is what we shall have to do temporarily till more is known of 

 the relations of these old rocks. 



But if neither of these alternatives be acceptable, can they be 

 classed with the Cambrian ? I shall not, with our present infor- 

 mation, attem[»t to answer this question definitely, but only throw 

 out a few suggestions. In the first place, the behaviour of the over- 

 lying quartzite does not prove that they cannot properly be called 

 Cambrian. Ou the one hand, it is verj^ probable that the grits 

 which lie upon them are also overlain by the quartzite, and yet thai 

 this grit is Cambrian. On the other hand, these conglomerates and 

 grits, though derived in part from them, need not be of much later 

 date, considering the rapidity with which volcanic rocks consolidate 

 and are broken up, and considering the connection, as noted above, 



