Dr. C. CaUau-ay — Sow to Work the Archcean Eods. 425 



group into the likeness of an older deposit. As previously urged, 

 we must compare formations, not piecemeal, but as a whole. 



It is scarcely necessary to utter a caution against confounding 

 regional with contact metamorphism. The change produced by an 

 intrusive mass of granite or greenstone rapidly dies out as we recede 

 from the point of contact, and is readily distinguished in the field 

 from the metamorphism which uniformly affects large rock-groups. 

 Contact alteration may be found in any formation into which igneous 

 rocks have been intruded, and it therefore affords us no aid in 

 Archffian work. There would also appear to be no d priori reason 

 why regional metamorphism should not also occur in rocks of any 

 age, and in the above remarks it is only contended that there are 

 some grounds for constructing an empirical rule, applicable only, so 

 far as present observations go, to a certain area. When the nature and 

 causes of metamorphism are more thoroughly understood, it may be 

 possible to reach a general principle. 



Dr. Sterry Hunt, of Montreal, has very ingeniously attempted, in 

 his " Chemical and Geological Essays," to prove that the Archaean 

 rocks of North America display a progressive chemical and mineral 

 change from the oldest to the youngest ; so that the minerals of a 

 group, like fossils in newer deposits, are indices of its geological 

 age. The writer does not here express an opinion upon this hypo- 

 thesis, as it is his design to confine himself to reasonings based upon 

 facts of which he has had personal cognizance. 



In correlating Archeean groups, it is important to ascertain the 

 origin of the deposits. In the Pebidian at St. Davids there is 

 abundant evidence of contemporaneous vulcanism ; indeed, the series 

 is mainly built up of volcanic ashes and breccias. The same origin 

 is attributed to the Bangor group, to the younger of the two 

 Anglesey series, to the newer of the two Archaean groups at Malvern 

 and in Shropshire, and to the Charnwood Forest rocks. We must 

 not, of course, press this argument too far, but it adds strong con- 

 firmation to other proofs. It is much weaker in the last case than 

 in the others, because by superposition the Charnwood series can 

 only be proved to be pre-Carboniferous. In all the examples, the 

 Arvonian of Dr. Hicks introduces complexity. It is of volcanic 

 origin, at least in great part ; but it is not yet certain to what 

 horizon it is to be referred — to the Dimetian, to the Pebidian, or to 

 a period between the two. It is also doubtful if all the rock-groups 

 which have been called Arvonian ai'e of the same age. So far, at 

 least, as Anglesey is concerned, the Arvonian has no independent 

 existence, but is simply a band in the gneissic series. In this area, 

 the (supposed) Dimetian also displays the characters of a volcanic 

 rock, where the alteration has not gone far enough to obliterate the 

 original structure ; but this need not prove a practical diflSculty, as 

 the more intense metamorphism of this group as a whole will readily 

 distinguish it from the Pebidian. 



The microscope is a most valuable auxiliary in Archaean geology. 

 It cannot supersede field-work, but it gives precision to the deter- 

 minations of the pocket-lens, and it sometimes ascertains structure 



