I. Introduction Much has been written about the exception rocks of the Ardennes, but the petrographical work has been treated, in some memoirs, apart from the field-evidence. This, indeed, owing to the limited occurrence of the most peculiar specimens, is not easy to obtain. It seemed then that a detailed study of a few interesting examples of the rocks, and of their relations in the field, might be worth a brief record. Further, eminent authorities have expresses different opinions as to the cause of the alterations. André Dumont in his famous monograph 2 described numerous observations, and inclined to the view that the structures were the result of contact-action, Prof. Barrois, 3 from comparison of specimens with those of Britanny, supported this theory. A. von Lasaulx 4 added the important evidence that a granite occurs in the Hohe Venn. Other authorities, however, have described the folding, contortion, and faulting of the rocks, and attribute the changes to mechanical influences. This opinion is advocated by Prof. Renard in a valuable petrographical paper, 5 and by Prof. Gosselet in his exhaustive memoir on the district. 6 Thus I became interested in the question, and, at the desire of Prof. Bnney (as he was unable at that time to undertake the investigation) I visited the district in 1898, and collected many specimens. After careful study of these, I made a second expedition in the summer of 1899, and gained observations over a wider area of country. In the course of this investigation I have received much valuable help