24 A. J. Jukes-Browne — Meilard Reade's Mountain Building. 



" On the Physical Conditions under which the Cambrian and Lower 

 Silurian Rocks were probably deposited over the European Area," 

 in the Q.J.G.S. for 1875, wliich has been reproduced by Mr. Walcott, 

 along with the map and sections for other European areas, in his 

 monograph. The main lines of division and zones remain as given 

 in that section, but it has since been possible to make several im- 

 portant sub-divisions and to add new zones.^ 



The information obtained since the Map was published has con- 

 firmed the view I expressed in the paper that wherever the base line 

 of the Cambrian is seen throughout the European area, it is almost 

 invariably found to " rest unconformably upon an earlier series of 

 rocks," and that the beds varied in thickness and character in ac- 

 cordance with the unevenness of the old pre-Cambrian floor and 

 along fairly well-marked lines of depression. It is not, of course, to 

 be expected that there should be an unconformable break between 

 the Cambrian and Pre-Cambrian in all parts over a very large area, 

 and Mr. Walcolt shows that there is conformity at that horizon in 

 several districts in America. In future the faunas must determine 

 the position where there is any doubt as to the presence of a physical 

 break. In Britain fortunately there is evidence of a very marked 

 break at the base of the Cambrian in all the areas examined. 



IV. — Ebade's Theory of Mountain-Building. 

 By A. J. Jukes-Browne, B.A., F.G.S. 



ME. MELLAED EEADE'S book on the " Origin of Mountain 

 Eanges " has now been before the geological public for five 

 years; it has been reviewed in this Magazine and elsewhere, and 

 several more or less weighty objections to the theory have been put 

 forward ; but no very complete examination of it has been attempted. 

 This may be partly due to the manner in which Mr. Reade has pre- 

 sented his theory, for he has certainly been too diffuse over points 

 which require very slight illustration and not nearly explanatory 

 enough on other jioints of great physical difficulty. 



Having had occasion to pay some attention to the subject, it has 

 seemed to me that some of the difficulties raised by some of his 

 critics are not so great as they imagine, while there are other diffi- 

 culties which neither Mr. Eeade nor his critics have sufficiently 

 considered. It may seem rather presumptuous on my part to 

 enter a field of controversy where such men as 0. Fisher, C. Davison, 

 and M. Eeade, are the debaters, and where the chief weapons used 

 — physics and mechanics — are such as I have small acquaintance 

 with. But some of the questions dealt with can be handled without 

 more than an elementary knowledge of these subjects, and I fancy 

 there are many geologists who wish to know exactly how far Mr. 

 Eeade's theory can be accepted as a vera causa. In what follows, 

 therefore, I am only endeavouring to assist in forming a conclusion 

 on this point. 



1 See "The Classification of the Eozoic and Lower Paleozoic Eocks," Popular 

 Sci. Rev. 1881, and " Becent Researches among Lower Palaeozoic Rocks," Proceed. 

 Geol. Assoc, vol. vii. 



