32 HOW TO WORK IX THE ARCH.EAN KOCKS. 



similar to the Wrekin series, 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 dozeu miles distant, is incomparably stronger 

 than the proof of the contemporaneity of the Bangor series, 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 tlie Cambrian. 

 The Charuwood sJaty series, for example, can only by superposition 

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

 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 impoi'tant 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 metamorpliism of a rock-gi'oup is an important factor 

 in the evidence. Recent investigations favour the conclusion that, at 

 least in England and Wales, all metamorphosed or partly altered 

 groups are Archsean, 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 metamorphism which may be 

 discovered would probably be Archjean. 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 felspathic rock, though of less antiquity, might be 

 intensely metamorphosed. We must also carefully distmguish 

 between regional and contact metamorphism. The presence of the 

 latter would be of no assistance in our work. 



In correlating Archaean groups, it is important to ascertain the 

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

 sedimentary or volcanic, or both. If successful in this point, we 

 narrow the issue to be decided. 



The viicroscope is of great value in these investigations. It gives 

 precision to field work, and discovers facts which field tests are not 

 competent to ascertain. 



