214 TIOW TO WORK IN THE ARCH/FUN ROCKS. 



Pebidian of St. David's. Indeed, this test is our chief guide amidst 

 the complexities of these old rocks, and its use has led to some of our 

 most interesting discoveries. Due caution must, however, be exercised 

 in its application. 



This kind of evidence decreases in value as the formations com- 

 pared increase in distance. There are, for example, volcanic rocks 

 similar to the Wrekin scries, both at Pontesbury near Shrewsbury, 

 and near Bangor. But it is obvious that the evidence for the 

 correlation of the Pontesbury rocks with the Wrekin group,from which 

 they are but a dozen miles distant, is incomparably stronger than the 

 proof of the contemporaneity of the Bangor scries, which is separated 

 from the Wrekin by the breadth of North Wales. 



In estimating the value of this test, it is first of all necessary to 

 ascertain if the rocks under investigation are older than the Cambrian. 

 The Oharnwood slaty series, for example, can only by superposition 

 be proved to be pre-carboniferous ; and the evidence for its Pebidian 

 age is, therefore, much weaker than the proof adduced for the Anglesey 

 slaty series, which is proved by included fragments to be Pre- 

 Cambrian . 



An important accessary test is similarity of succession. Thus the 

 green schist on the Menai Straits closely resembles the rock underlying 

 the granitoidite in the centre of Anglesey. But it would be just 

 possible, were there no other evidence, that the resemblance was a 

 mere coincidence. The discovery, however, of a grey gneiss under the 

 green schist in both areas removes the doubt; for that the similar 

 succession should be due to coincidence is simply incredible. 



In comparing formations by their mineral characters, it is requisite 

 to take them as a whole. Rock specimens selected with a view to 

 favour a preconceived theory will lead to the most bewildering results. 



The degree of metamorphism of a rock-group is an important factor in 

 the evidence. Recent investigations favour the conclusion that, at 

 least in England and Wales, all metamorphosed or partially altered 

 groups are Archaean, and that the intensity of the alteration is 

 proportioned to the age of the series. All the cases of metamorphic 

 Cambrian and Silurian, adduced by Murchison and others, have, on 

 re-examination, broken down, and there would appear to be some 

 grounds for constructing an empirical rule that, within the above 

 limits, any new area of regional nietamorphism which may be 

 discovered would probably be Archaean. Numerous proofs of these 

 points might be adduced, if space permitted. It is, of course, 

 necessary in these studies to bear in mind the influence of selective 

 metamorphism. A quartzose rock, for example, can undergo little 

 change; whereas a telepathic rock, though of less antiquity, might be 

 intensely metamorphosed. We must also carefully distinguish 

 between regional and contact metamorphism. The presence of the 

 latter would be of nq assistance in our work. 



In correlating Archaean groups, it is important to ascertain the 

 origin of the deposits ; whether, for example, the rocks were 



