MONIA.N SYSTEM OP ROCKS. 485 



probably intermediate ia age between these two. The Ordovician 

 basement-beds are found also at Holyhead, where they lie on the 

 quartzite, at Garn, where they lie on the pelite, and at Pen-bryn- 

 yr-Eglwys, where they lie on agglomerate. Nowhere is there found 

 any rock like the Cambrians of Bangor. The whole series is thus 

 proved to be a continuous Pre-Ordovician one, with two different 

 developments in an upward direction. 



The Cej^tral District. 



This district is the largest and most difficult to understand. In 

 it Dr. Callaway thinks he finds representatives of all the subdivisions 

 of his gneissic series. My principal difficulty is with the rocks 

 which he calls " Halleflinta " ; on these I have twice changed opi- 

 nions, now reverting to that which the field-work suggested. It 

 may be wrong, but, if so, the stratigraphy is almost infinitely 

 complicated. According to this we must take particular notice of a 

 line which is marked on the Survey map, and which runs from 

 Porth-y-ly-wod on the coast nearly straight to the eastern mass of 

 granite at Gualchmai, and thence to Bodwrog church. This is a 

 line of fault. Its further course is doubtful, since in any case 

 there must be a second disturbance beyond the railway. This line 

 divides the district into an eastern and a western region. 



■'&* 



The Eastern Region". — The basal rock in this region, occurring 

 next to the fault, is what Dr. Callaway calls quartz-schist, and it is 

 succeeded by a type of rock which we have not seen before, to which 

 he gives the very descriptive name of the grey gneiss, inasmuch as it 

 contains felspar as well as mica and quartz. I can see no necessity 

 for distinguishing a quartz-schist, since there is felspar, though less 

 abundant, in the most westerly samples. The rock is, for the most 

 part, more thoroughly metamorphosed than any in the western 

 district, so that it is difficult to distinguish any lines of original dust. 

 The foliation, however, is very fine and well marked, not only by 

 the chlorite but by the long axes of the other minerals. Dr. Callaway 

 has given a good description of the development of these rocks from 

 Perth Nobla to Aberffraw, which need not be repeated at length ; 

 suffice it to say, that more quartzose bands and more chloritic bands 

 come on irregularly, the latter especially on the eastern side of 

 Llangwfen Bay ; but everywhere the beautiful foliation is well 

 marked, and nowhere better than when the series is about to close 

 on its meeting a fault close to the western promontory of Aberffraw 

 Bay. For some distance here the lines are horizontal, but on ap- 

 proaching the fault they become wonderfully contorted. The rock 

 is easily traced inland, as in the railway-cutting and to the north of 

 it, and so on either side of the stream at Gualchmai up to the 

 granite, and beside Bodwrog church and the old Holyhead road. 

 The limestones, which have been noticed at Trecastle and Bodwrog, 

 and which are truly foliated with the rest of the gneiss, are impor- 

 tant evidence of the unity of the series. Like those around the 



