486 EEV. J. r. BLAKE ON THE 



serpentine of Valley, they are in the older portion, but they have 

 the same sporadic character as those in the younger. 



In the correlation of this group with the western district I quite 

 agree with Dr. Callaway. Though the metamorphism is more com- 

 plete, and the ingredients more minute, they are of the same cha- 

 racter as the chloritic schists. Their more quartzose base and their 

 more chloritic termination show that they must be conceived to cor- 

 respond generally to the whole of the lower group, and no marked 

 line can be drawn between grey gneiss and chloritic schist. 



In all the district northwards from within a mile of Gualchmai, 

 the eastern boundary of the gneiss and associated rocks is formed 

 by basal Ordovician conglomerates and grits, which, at the crossing 

 of the old Holyhead road, form a beautiful white rock, and these 

 are soon followed by the ordinary black shales standing at a high 

 angle. Still, therefore, no Cambrian rocks intervene, but the older 

 rocks were denuded to their base in Cambrian times. These Ordo- 

 vician shales are bounded on the east by a well-marked fault, bring- 

 ing in again the Pre-Cambrian series. We cannot suppose that this 

 important fault dies out exactly where it cuts off the Ordovician and 

 brings the two portions of the Pre-Cambrian series together, as it 

 has hitherto been inf erentially represented as doing ; yet there is this 

 strange circumstance about it, that it represents a downthrow on 

 the east as against the gneiss, and an upthrow as against the Ordo- 

 vician. There are only three ways of accounting for this : either 

 (1) the grey gneiss is really the younger rock (an untenable suppo- 

 sition), or (2) the fault coincides with the bedding-planes, which 

 it does not, or (3) there was a Pre-Ordovician as well as a Post- 

 Ordovician fault along the line, with opposite throws, which shows 

 that the fault must be a well-marked one, but the balance of displace- 

 ment need not be great. The continuation of the fault to the south 

 can be traced on the Holyhead road between the ^ milestone and 

 the slopes of the hill by Glan-gors-du Fawr. Two miles to the 

 south it passes between Tyn-y-gong and Cwlrwm, and is next seen 

 in the railway- cutting near Bodgedwydd. A wall is built over it 

 on the south side, all the rest of the cutting being rock. But on the 

 north side solid green rock on the east comes suddenly to an end 

 on a nearly vertical line, next to which is a foot or two of broken 

 rubbish, and then we find thin-bedded micaceous rocks of the grey- 

 gneiss series, which continue for the rest of the cutting. I give 

 these details because Dr. Callaway says there is here a passage from 

 the grey gneiss into the " dark schist," and the fault runs much 

 further east, where it cannot be traced. Thence the fault passes 

 west of the road to Aberfi'raw, near the church and west of the small 

 promontory of Trwyn-du, where the rocks on the two sides are in 

 strong contrast. 



We are thus prevented from following the sequence upwards, and 

 must start afresh with the rocks on the east side of the fault. It is 

 not easy to make certain of their stratigraphy. Dr. Callaway 

 divides them into two independent halves by a fault which passes 

 from the AberflEraw sands into the eastern boundary of the Ordovician 



