MONIAN SYSTEM OF ROCfiS. 497 



latter. It is found in the centre between the two granite tongues 

 at Llanfaelog, where it is called " Silurian ? " in the Survej^ map, 

 and skirts the east side of Llyn Paelog. It also lies next the 

 granite near Tycroes, and is found in patches between Llanfaelog 

 and Pen-y-carnisiog. It forms the tongue of "Cambrian " that cuts 

 the Holyhead road and into which the diorite intrudes further 

 north. Near Bodwrog church it lies between the hiilleflinta and the 

 grey gneiss, being separated from the latter by the granite. The 

 rock that forms the long tongue north of Gwyndy is more of this 

 type than any other, and it occurs on the other side of the granite 

 near Llanerchymcdd windmill. The name, however, is an inclusive 

 one for all somewhat similar rocks which have no special character. 

 On the whole it is found more to the west than the hiilleflinta, but 

 at Bodwrog it is next the grey gneiss. 



3. The Gneisses. We find these, apparently graduating into the 

 pelites, near Llecheynfarwy. They have here a banded character. 

 Also near the farm of Mynydd Mawr the rocks are very quartzose 

 rough gneisses not in the least like the grey gneiss. This type of 

 rock is continued to Tafarn-y-botel, where the granite intrudes, 

 and is the most difficult of all to understand. At the latter 

 place it is a beautifully clean crystallized rock, composed of quartz 

 and felspar quite fresh, and black mica, which has in part passed 

 over into another mineral. There is nothing like this rock in the 

 whole of Anglesey elsewhere, and the question is suggested whether 

 we may not touch here a piece of genuine Archaean. The nearest 

 rock to this in character occurs on the north-east near Plas Llanfi- 

 hangel. In connexion with this may be mentioned a mass of rock 

 in the centre of the granite, on the railway between Llangwllog and 

 Llanerchymcdd, which is more like grey gneiss, with an extraordi- 

 nary amount of white mica and sericite, amounting to half its 

 volume, and with which a calcareous rock is associated. 



4. The Diorites. This is the only name I can satisfactorily 

 give to the rocks which in many places are hornblende-schists. 

 They appear partly as "gneiss" and partly as " greenstone " in the 

 Survey map, and they figure in Dr. Callaway's descriptions as " dark 

 schists." Sir A. Ramsay regards them all as metamorphic sedi- 

 mentary rocks. jN'o doubt their foliated appearance suggests sedi- 

 mentation ; but I find that so long ago as 1872, in Jukes's ' Manual 

 of Geology,' it is stated of hornblende-schists that '' there is reason 

 to believe that as they occur among altered sedimentary rocks 

 they may represent former trap-rocks ;" and of recent years this 

 conclusion has been verified by \Yilliams in America, and by Teall 

 in this country. It was not, however, entirely their foliation which 

 led Sir A. Eamsay to his conclusion, since he places in the same 

 category in the memoir the unfoliated rock which is coloured green- 

 stone on the map, but it was rather the shading off of both this 

 and the granite into the surrounding rock which convinced him that 

 the one was the derivative of the other. It is for the microscope 

 to decide which is the original rock. If it was the sedimentary one, 

 then as we approached the crystalline, isolated crystals would appear 



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