^Ol. 57.] GEOLOGY OF MTXTDD-Y-GAKN. 21 



5. Garn Breccia, Grit, and Conglomerate. 



6. Black Slates with courses of grit and breccia. 



7. Black Slates without grit-courses, but with layers of tough mudstone. 



8. Llanfair-y'nghornwy Beds. 



9. Green Series of Northern Anglesey. 



As the traverse follows the direction of dip there is apparently 

 an upward succession all the way; the Black Slates and grit of 

 'Craig-y-gwynt seem to be at the base, and the rocks of the Green 

 Series of the Northern District at the top. The true sequence of 

 ihe rocks is, however, quite different from this apparent order. 



The following descriptions should be read in connection with the 

 map (fig. 2, p. 24). It will be most convenient to commence with 

 the green phj'llites at the southern end of the hill. 



Garn Phyllites. 



These are mostly green rocks of fine grain, apparently once shales 

 intercalated with gritty layers (sometimes sufficiently differentiated 

 to form flaggy bands) ; but they are now sericitic and chloritic 

 phyllites, often well though minutely foliated, and usually containing 

 Silastic material. In their lower part they are almost exclusively 

 green, though in one spot near Nant, close to where they are faulted 

 against Black Slates, some purple zones appear in them. At the 

 southern part of the hill they are picturesquely contorted, and are 

 indeed as highly ' gnarled ' (Hughes) as in the better-known area 

 «ast of Amlwch. Though the crumpling has usually been effected 

 with ver}' slight disruption, the rock is occasionally found to have 

 been brecciated in the process. The ' gnarling ' appears to be of 

 later date than the foliation, and seems to be the result of post- 

 Llandeilo movement. 



The contortions become less pronounced northward, and die out ; 

 the green phyllites give place to rocks which are less distinctly 

 green, being more often bluish or brown. They consist of compact 

 phyllites and fine grits, and as a rule are less fissile than those 

 just described. The dips are northerly, or veer between north- 

 north-west and north-north-east, and measure from 45° to 80°; but 

 the planes which determine the dip are perhaps cleavage- or shear- 

 planes, and may not represent true bedding, for the structures of 

 these phyllites when examined under the microscope are seen to be 

 largely secondary, the result of dynamic action. Three microscopic 

 sections [N.A. 138, 139, & 140 ^] have been sliced from these rocks 

 and all exhibit brecciation in situ, slide iJ^.A. 140 having the 

 character of a thrust-conglomerate. 



The green gnarled rocks are easily correlated with beds of the 

 Oreen Series of the Northern District of Anglesey, and the upper 

 beds must also be grouped as part of the same series. 



These phyllites are cut off to the west and south by a strongly 

 curved fault which brings them against Black Slates on the west, 



^ The numbers in square brackets are those of the slides in my cabinet. 



