MOlflAN SYSTEM OF ROCKS. 517 



dust is more or less mingled with angular fragments of apparently 

 volcanic origin, indiscriminately introduced. 



The sporadic and intrusive rocks associated with these sedimen- 

 tary deposits are of considerable interest. Among the former, we 

 find that the mass marked " greenstone," to the south of Llanfechell, 

 is partly serpentine and partly a brecciated purple limestone, once 

 worked as Mona marble. Another mass, marked " serpentine," at 

 Tregela, is no longer exposed ; and the only rock that can be found 

 there is a solid purple limestone like that of Cerrig Ceinwen. I 

 have found no quartz-knobs in this portion of the series. There 

 are also numerous intrusive dykes of both basic and acid character. 

 Among these are the " greenstones " and " felsites " in the west, as 

 well as sundry isolated bosses, not marked upon the Survey map. 

 Most of these may be referred to a later portion of the same period, 

 partly because they cease at the fault, and partly because, in the 

 case of the felsites, they show peculiarities of structure similar to 

 those of known Pre-Cambrian eruptions. Some of them may, how- 

 ever, be of the age of the Parys-Mountain felsites. They cannot 

 here be described. 



The distfebed Volcanic Group. — This is by far the most inter- 

 esting portion of the district. The rocks composing it must have 

 been formed during a period of much greater activity, either volcanic 

 or disturbing. The line of separation between these and the more 

 sedimentary facies is much more clearly marked on the west than 

 on the east. A rapid and almost sudden change may be observed 

 when we pass a line which cuts off the northern corner of the pro- 

 montory on the east side of Camlyn Bay, and then runs from Porth- 

 y-gwarthog near the shore to the north side of Cemmaes pier, thence 

 near the Amlwch road, and so on to Perth Llechog on Bull Bay. 

 The district north of this line may be roughly described as full of 

 agglomerates, coarse ashes, quartz-knobs, limestones, and conglo- 

 merates. 



Beginning at the west, we find agglomerates on the headland of 

 Trwyn-pen-careg. In the next bay a return is made to the slates, 

 but in the succeeding headland north of Porth-y-gwarthog agglo- 

 merates recommence, and ultimately in Porth-wnal there is wild 

 confusion in the rocks. The most remarkable feature is the occur- 

 rence of great quartz-lumps, which are of all sizes and shapes, and 

 lie promiscuously in agglomerates of slates, grit, and dust; and 

 the whole is intruded upon by greenstone dykes, which are both 

 banded and prismatically jointed. These lumps appear to have 

 been thrown into their present position; but, as there is nothing 

 like them in the neighbourhood except those which occur under 

 similar circumstances, we must seek their first origin not far from 

 their present site. They must have been in existence previous to the 

 formation of the agglomerate, which we cannot here refer to the 

 action of a crush-fault. The nearest quartz-knob, which, like all 

 the others, is quite isolated, is at Mynydd Wylfa. In Porth-yr- 

 wylfa is a long tongue of limestone, ending off roundly in the 



