422 Dr. C. CaUaicay — Soiv to Work the Archcean Bods. 



But there is the bare possibility, though little probability, that the 

 former is some other unknown group, since the strata of the two 

 districts cannot be connected with each other, owing to an inter- 

 A'ening faulted area. Further examination, however, removes the 

 shadow of the doubt. Underlying the dark schist in the Menai 

 band is a grey gneiss precisely similar to the beds (d), which pass 

 under the green schist (e) in the central area. We have here, then, 

 a resemblance not only of mineral composition and structure, but of 

 succession. It is absolutely incredible that similar gneiss underlying 

 similar schist in the two localities, which are only three miles apart, 

 should be due to coincidence. 



In comparing formations by their mineral characters, it is neces- 

 sary to take them as a wJiole. To select specimens with a view to 

 favour a preconceived theory will lead to no satisfactory result. 

 Some examples will illustrate this point. Comparing the Archaean 

 volcanic series of Shropshire with the Archasan volcanic series of 

 Bangor, we find some striking resemblances. Some of the felspathic 

 grits are almost undistinguishable in hand specimens ; colour, texture, 

 and constituents being the same. On the other hand, the differences 

 are numerous. The lava flows of Bangor contain blebs and small 

 crystals of quartz, and many of the grits and slaty beds are unlike 

 in colour and composition. These dissimilarities are by no means 

 fatal to the identity of the two groups, but they weaken the evidence 

 in its favour. A second example occurs in the Scottish Highlands. 

 One of the great unsettled questions of the day is the age of the 

 newer gneiss series, which occupies the chief part of northern and 

 western Scotland. Murchison claimed to have proved its Ordovician 

 (Lower Silurian) age, but some others have asserted that it is simply 

 the fundamental gneiss or Lewisian of the north-west coast brought 

 up again in the centre and east of the Highlands. Those who hold the 

 latter view maintain that the specimens which they have obtained 

 from the Lewisian are closely similar to some of the eastern types. 

 This may be quite true. Yet it is indisputable, as pointed out long 

 since by Murchison and his followers, that, taken as a whole, the 

 two rock-groups are widely dissimilar. The older gneiss is massive, 

 intensely crystalline, and frequently hornblendic; while the younger 

 is thin-bedded, not coarsely crystalline, and contains mica instead 

 of hornblende. While, then, in this district it may be difficult here 

 and there to assign a rock to its true place on lithological evidence 

 alone, yet, studying the rock-masses on the large scale in the field, 

 the differences between the two gneisses are broad and well-marked. 

 Whether or not Murchison was right in claiming an Ordovician age 

 for the thin-bedded gneiss is another question ; but, in the writer's 

 opinion, there is not a shadow of doubt that it is quite distinct from 

 the Lewisian, and of less antiquity. The Pebidian rocks of Anglesey 

 furnish us with another illustration of the necessity of comj)aring 

 formations as a whole. In some parts of Northern Anglesey, the 

 slaty rocks are highly altered, so as here and there to put on the 

 form of a true metamorphic schist, which approaches in texture and 

 composition some of the chloritic bands of the older gneissic series. 



