Reviews — Whitney and Waclsicorth'' s Azoic System. 29 



Our authors deal very trenchantly with Dr. Hunt's conclusions. 

 The Norian, Arvonian, and part of the Huronian are relegated to the 

 category of intrusive igneoiis rocks. The rest of the Huronian is 

 regarded as a variety of things, but usually as altered "Silurian." For 

 the Keweenian a Potsdam age is positively claimed. The Montalban 

 and Taconian are either a part of the (so-called) Laurentian or 

 metamorphosed Palaeozoic rock. Nothing is left, but a confused 

 mass of strata, to vi^hich the general name of " Azoic " is applied. 

 If our authors are right, the Archsean regions are almost a terra 

 incognita. Few true results have been obtained, because false 

 methods of work have been adopted. 



One of the fallacies which, in the view of Messrs. Whitney and 

 Wadsworth, has misled Logan and his followers, is the assumption 

 that the so-called metamorphic rocks were deposited as aqueous sedi- 

 ments, and that the present foliation corresponds with the original 

 bedding. Our authors do well to express this caution. Igneous 

 rocks, which have by pressure acquired a foliated structure, have 

 sometimes been described as metamorphic, and identified as Archajan 

 formations ; but modern woikers are becoming aware of the danger 

 of deception, and a good field geologist will rarely be led astray. 

 Part of the old work will probably require revision, and some of 

 the " Archaean " masses may fall back into their old state of 

 " granite " or "greenstone." But we apprehend that the American 

 and Canadian Archesan geologists will hardly admit that the exist- 

 ence of pressure foliation in igneous rocks is a fatal hindrance to 

 the success of their work. 



Our authors also wisely call attention to the possibility of error in 

 using the test of included fragments. They insist upon the fact — 

 familiar enough to geologists — that in volcanic formations, included 

 fragments may be the result of contemporaneous denudation. They 

 assert that the so-called Keweenian (Keweenawan) system owes its 

 existence to the blunder of supposing that conglomerates in Potsdam 

 rocks prove the superior antiquity of the masses from which the 

 included pebbles were derived. Other errors of the same kind 

 have, in the view of our authors, seriously invalidated many of the 

 conclusions of Hunt and his co-workers. On these criticisms, with 

 the evidence furnished in the book before us, we can say notliing. 

 Each case must be tested on its own merits, and an opinion can 

 only be formed on full lithological descriptions, confirmed by the 

 microscope. 



It was to be expected that our authors would make a point of 

 Dr. Hunt's frequent change of opinion. But it is only fair to Dr. 

 Hunt to state that these changes have usually been in one direction. 

 Territory after territory has been absorbed into the Archaean empire, 

 but we do not remember any case in which the conqueror has re- 

 stored any of his annexations. 



British geologists will be rather surprised to learn that even Sir 

 Eoderick Murchison betrayed a " want of tenacity of opinion " on 

 the Archaean question. Our authors, however, accuse him (p. 521) 

 of this weakness, simply because he recognized the Laurentian and 



