MONIAN SYSTEM OF ROCKS. 531 



almost tempted to believe that the chloritic schists are reached, but 

 the rock when examined is still very ashy. There are numerous 

 dykes marked on the map, to which 8ir A. Ramsay calls attention. 

 All of these that I could identify seemed not to be of later date, 

 but true associates of the ashes, being the more crystalline portions 

 and, in some cases, perhaps actual flows, in others due to infiltration. 

 In particular the masses marked "serpentine" are for the most 

 part diabases with associated hard bands, which are often calcareous. 

 This is the case, as has been shown by Prof. Bonney, at Porthdinlleyn, 

 BO it is at Trefgraig, and at Ty-hen, near Bodferin. The masses 

 further south are very much mixed and include several kinds of 

 rock, some of which are calcareous, but all are such as we might 

 expect to form part of basic eruptions. On the higher parts of 

 Mynydd Anelwog they are more siliceous and might bo called fel- 

 spathic ashes. The centre of eruption seems to have been at the 

 southern extremity of the peninsula, to judge from the enormous 

 size of some of the ejected blocks. The headland of Uwch Mynydd, 

 as seen from the sea, is a mass of large agglomerates with calcareous 

 and quartzose patches, such as may be seen on the northern coast of 

 Anglesey. Similar rocks to these I have been able to examine on the 

 eastern side of Bardsey Island, and there is no doubt of their cha- 

 racter : they are often in contorted beds, and some parts in their 

 midst are quite slaty ; on the western side the slates prevail and are 

 of purple and green colours ; amongst them, on the slope of the hill 

 and by the lighthouse, are low quartz- knobs of the usual isolated 

 form and characteristic structure ; limestones also occur sporadically 

 on the east. There is much more that is of interest in this portion, 

 at which I have only had time to glance. 



The great mass marked " syenite," to the east of Llangwnadl, and 

 another to the south, I believe to be continuous. Nothing is to be 

 seen between the two masses in the peat-covered ground, and a 

 " feature " seems to pass from one to the other. The southern 

 mass does not extend so far as marked, but the higher slopes of the 

 hill are occupied by a massive diabase, which rises into a little 

 isolated peak called Clip y cefinhir, which is a perfect picture of a 

 volcanic neck, with beautiful columns, sloping sharply to the east. 

 The figure of " Alesna " in the Mount Tabor region, given in the 

 6th Annual Eeport of the United States Geological Survey, might 

 well stand for its portrait. Nor is this mass certainly separated from 

 the ashy group by a corner of the Ordovician, as marked on the Survey 

 map, since no rocks are exposed in the interval, which is low and 

 marshy. Where this mass is seen in the neighbourhood of the ashes, 

 its relations appear to be those of an intrusive rock, since the latter 

 become more micaceous and altered near the junction, but this is 

 not actually seen. It thus becomes an assumption, so far as strati- 

 graphy is concerned, that the mass is of Pre-Cambrian age, even 

 admitting that the ashes are. If it be right to consider it so *, it 



* These words stand as they were wi-itten, Dec. 1887. Mr. Harker has since 

 shown that the granite is intrusive also into the slate rocks to the east, and 

 cannot therefore be Pre-Cambrian. 



