Vol. 55.] GEOLOGY OY NOETHERN ANGLESEY. 637 



II. Desoeiption of the Rocks. 



By far the greater part of the area is occupied by the set of strata 

 called by Prof. Blake the ' Sedimentary Series/ in contradistinction to a 

 ' Disturbed Volcanic Group ' which forms a strip in the northernmost 

 part. As, however, this disturbed group appears to be as sedimentary 

 as the former, I use the term ' Green Series ' for the prevailing strata 

 north of the Ordovician boundary, because they are characteristically 

 of a greenish tint, though they have purple and yellowish bands in 

 them. The ' Disturbed Volcanic Group ' of Blake will for the present 

 be called merely the ' Northern Complex.' 



(a) The Green Series. 



A careful and graphic description of these rocks is given by 

 Kamsay in the ' Geology of North Wales,' the principal correction by 

 later writers being to limit his use of the term schist. The strata 

 on the whole dip to the north or, in the western part of the area, to 

 the north-east; but these directions are not constant, especially in the 

 western part. For instance, in the neighbourhood of Pen-yr-orsedd, 

 Caerau, and Nanner the beds have a south-easterly dip, and near 

 Groes-fechan they form a low dome. Various writers have pointed 

 out the characteristic types of rock. The highly contorted M y n y d d 

 Mechell Schists occupy the centre of the curve of the boundary- 

 fault ; in part they are thoroughly foliated and crystalline, in part 

 they are very quartzose and distinctly gritty, and they appear to be 

 grits and shaly beds in which a crystalline texture and foliated 

 character has been developed, sometimes to the complete obliteration 

 of clastic structure. Dr. Callaway has recently shown that the 

 foliation is more marked and the crystalline structure more complete 

 as we pass southward to the boundary-fault.^ I am inclined to 

 think that they are in part foliated representatives of the Llan- 

 fechell Grits, a set of rather thickly -bedded green grits which 

 succeed the schists to the north. 



With the exception of the beds in the extreme north of Anglesey, 

 the country north of the Llanfechell Grits is occupied by green 

 slates with purple zones and numerous coarser gritty layers. The 

 Rhosbeirio Shales of Dr. Callaway — soft greenish, purple, and 

 yellowish-brown shales in the centre of the district, with a constant 

 northerly dip — form part of this series ; and similar beds may be 

 followed across the strike as far as Cemaes. To the east of 

 Amlwch they are extremely contorted or ' gnarled.' Grits (of 

 a greyer type) again set in in some force in what appear to be 

 the highest beds, along part of the northern coast, especially in 

 the neighbourhood of Trwyn Cemlyn and Cerig Brith, and they are 

 recognizable among the contorted beds near Point vElianus. 



It is generally considered that these slaty and gritty deposits 

 contain a certain proportion of volcanic dust and ash, and, though 



1 Quart. Journ. Geol. Soc. vol. Hv (1898) pp. 374-381. 



