MONIAN SYSTEM OF ROCKS. 515 



for not recognizing that in this Northern District the base corre- 

 sponds to some part, probably rather the upper part, of the long 

 chloritic series. The more or less quartzose rocks, such as those 

 that form the rugged slopes of Mynydd Mechell, and the finer sedi- 

 ment, more impregnated with sericite, constitute the two principal 

 types, irregularly distributed. 



These more crystalline portions extend northwards to about the 

 parallel of Bodewryd, near to Llanfechell, and south of Llanfairyng- 

 hornwy. It is true that a number of felsite and greenstone dykes 

 cro.s this area ; but rocks already crystaDine cannot be much altered 

 by such dykes, and in fact they make but little difference either one 

 way or the other. 



The first rock in the ascending series which we can usefully asso- 

 ciate with those above is the Llanfechell Grit of Dr. Callaway. This 

 is much more fragmentary, and compares best with the ashy rocks 

 seen at the crossing of the river Alaw, south of Llanfachreth. It 

 certainly contains what may be fragments which resemble certain 

 parts of rocks referred to the chloritic schists ; but we find such parts 

 in the rocks of Mynydd Mechell, and such fragments occur, though 

 of smaller size, in some of the chloritic schists of the western dis- 

 trict themselves. Some of these fragments, from their angular form 

 and variation in character and size, may be volcanic contributions, 

 others vein-substances ; but none are indubitably schists, and we need 

 not therefore suppose any unconformity, of which there is no strati- 

 graphical evidence, between the rocks of Llanfechell and those of 

 Mynydd Mechell. These grits are local and pass westward towards 

 Mynydd Ithall. 



JFollowing these, on the rise, is a considerable mass of slates, of 

 which some are ashy, some purple, and some sericitic and smooth 

 to the touch, with an occasional band of grit. This includes the 

 Rhosbeirio Shales of Dr. Callaway. They are well developed on the 

 northern coast, between Cerrig Dehisgryn and Camlyn Bay, where 

 they are more like blue Ordoviciau slates than any in the island, 

 except those in the South-Stack Series and at Careg Gwladys. At 

 the base they are coarser, and towards the top are several bands of 

 grit with indurated cappings seen near Camlyn Point. Some of 

 these, inland, are very silky, as at Eodewryd turret, and some have 

 angular fragments of quartz and felspar sporadically distributed, so 

 that volcanic contributions continued to be made. Similar rocks 

 continue to Hafod Onen on the east, where they are somewhat 

 altered by neighbouring dykes. It is here that Dr. Eoberts supposes 

 a passage to the Ordovician slates; but the rocks are very much 

 broken. 



The rocks which succeed are not very different in character, as a 

 whole, but are greener in tint and more compact. They form a 

 band which, commencing in Camlyn Bay, is three quarters of a mile 

 broad at Cemmaes, and continues past Amlwch to the headland of 

 Point -^lianus. When these are undisturbed they are singularly 

 like the ordinary Cambrian or Ordovician slates, though not like 

 such of the latter as are found in Anglesey itself ; and there seem 



