644 MR. C. A. MATLEY ON THE [A-Ug. 1899^ 



(b) Stratigraphical Evidence. 



Notonl)^ is the palaeontological evidence strongly against the Bala 

 age of the Green Series, but stratigraphical considerations are equally 

 conclusive, althouuh some writers have adduced evidence that part of 

 the boundary cannot be anything but a beddmg-plane. That the- 

 Green Series is older can be demonstrated in two ways : (i) by an 

 examination of the boundary, and (ii) by proofs of an unconformity.. 



(i) K'ature of the Boundary. 



The boundary is mapped by the ofScers of the Geological Survey 

 as a simple curved line, broken south of Amlwch by a cross-faulL 

 In so crumpled a region as Anglesey the simplicity of the dividing- 

 line is in favour of its being a dislocation. Eamsay ^ inferred the 

 necessity of a fault from the dying-out of the dykes at the junc- 

 tion, and Blake^ because the boundary brings together various parts 

 of the two series. As J)r. Callaway^ has already examined this 

 junction, especially with reference to the eastern half, I have con- 

 tented myself with examining it in that portion at only a few 

 localities, but have devoted more attention to the western part of 

 its course. At its eastern extremity, in the little cove of Perth y 

 Corwgi, it is admitted to be faulted, and the hade seems to be 

 steeply to the north. North of the break are the highly-crumpled 

 or ' gnarled ' green rocks ; south of it are Ordoviciau slates, grits, 

 and pebbly slates, which are so broken as to become in part a 

 ' crush-conglomerate.' The movement producing the dislocation 

 is therefore clearly one of compression. Dr. Callaway has pointed 

 out that conglomeratic Ordovician beds fringe the boundary for 

 about 2 miles ; he suggests that we are near the base of the Black 

 Slate Series, and that the Ordovician is here inverted. Prof. Blake 

 has examined the two localities farther to the south-west, where 

 Dr. Roberts'^ states that the Ordovician can be traced quite uninter- 

 ruptedly into the Green Series, but he does not consider that the 

 exposures justify the latter's conclusions.' JS'owhere did I find a 

 passage between the two series. At Bothedd, where a passage has 

 been suggested,^ the Ordovician black slates are interbedded with 

 bands of grit and breccia whose green colour at first sight suggests 

 a transition ; but these bands are very distinct from the apparently 

 overlying rocks, and owe their colour to the circumstance that they 

 are largely made up of green fragments such as are found in the 

 Green Series. A microscopic section (N.A. 49)^ of one of these bands 

 shows it to be a grit almost entirely made up of fragments of fine 

 phyllites, generally sericitic, a few quartz-grains, and some chloritic 

 shales. The clastic structure is not due to crush. The presence of 



1 Mem. Geol. Surv. 'Geol. N. Wales,' 2nd ed. (1881) p. 235. 



2 Quart. Journ. Geol. Soc. vol. xliv (1888) p. 514. ^ jSid, vol. xl (1884) p. 567. 

 * Geol. Mag. 1881, p. 573. 



5 Quart. Journ. Geol. Soc. vol. xliv (1888) pp. 515 & 529. 



6 Ibid. vol. xlvii (1891), Pres. Adclr. Proc. p. 132. 



''' The numbers in parentheses are those of the slides in my cabinet. 



