On the Production of Secondary Minerals at Shear-zones in the Crystalline Rocks of the Malvern Hills

Abstract

In an introductory paper * read before this Society in April 1887, I gave a rough sketch of the mode in which certain of the Malvern schists † were produced ; but no attempt was made to offer a complete solution of the questions discussed. At the British Association in 1887 and 1888, and at the International Geological Congress in 1888, I briefly stated certain additional results, reserving particulars of the evidence for more detailed communications. One of the most important branches of the inquiry is the genesis of the respective minerals, especially of the micas. If, as I hope to prove, some of the most abundant minerals are of secondary origin, the ground will be cleared for working out the details of the problem in hand. Minute particulars of rock-composition will rarely be necessary in this paper. The principal varieties of the Malvern rocks have lately been carefully described by Mr. Rutley ‡ , and, furthermore, I believe that the question is to be settled rather by field-evidence than by the study of minute structure. Indeed, the chief results were decided in the mind of the writer before a single slide had been cut. Nevertheless more than 150 rock-sections have been prepared. Some typical examples of these were submitted to the skilled examination of Dr. H. B. Patton, assistant to Prof. Rosenbusch, of Heidelberg, and he has generously permitted me to incorporate his notes in this communication § . My thanks are also duen to Mr. J. J. H. Teall for several valuable suggestions, and his admirable monograph * has been of much use. It is satisfactory to note that Mr. Teall has recognized † at the Lizard certain "banded gneisses" to which he assigns an origin similar to that of the "injection-schists" of my last paper ; but I understand that he expresses no decision on the genesis of the granite-veins. Prof. Bonney has also favoured me with his opinion on certain points.