PREFATORY NOTE. The inception of this little work is due to the Presidential Address of Professor Bonney to the Geological Society in 1886. When that Address appeared it seemed to me that some of the leading ideas contained in it would admit of a fuller consideration from the chemical and physical side and in this I was happy to find that the author of the Address concurred. My first attempt to deal with them was in a paper which I hastily put together for the Birmingham Meeting 1886 of the British Association. I found however that even in its incipient stage the subject was too vast to be dealt with satisfactorily in a paper, and I had to content myself with a brief statement of some of the leading points, which appeared in the Associations Eeport for that year. In writing the Thesis on Bock-Metamorphism I was fully conscious of many imperfections in the treatment of some portions of the subject. It was especially so with parts of Sections ii and iii, which were written for the most part in 1886. As the other parts and the subsidiary matters contained in ii Appendix grew to considerable proportions, I found that with my daily work and the inroad which two other papers made upon my time in 1887, I was not able to re-cast Sections ii and iii as I could have wished to do, without risking the delay of another whole year in sending in the Thesis, and this for obvious reasons it would have been unwise to do. These matters were worked out more fully in a supplement, copies of which have been privately distributed along with the Thesis. The Thesis has been submitted to some of the highest authorities in this country and on the Continent and the friendly acknowledgements it has met with abroadhave been to me encouraging in the highest degree. The matter contained in the Supplement is now published at the suggestion of the University Examiners incorporated with the original Thesis, which has undergone careful revision, the alterations being however to a great extent merely verbal. Some few further additions have been made both to the body of the work and to the original appendices and these together with the matter contained in See Q.J.G.S. for May, 1888. the Supplement have been printed in smaller type. A little delay in the publication has enabled me to draw attention here and there to valuable contributions to petrology made by foreign geologists of eminence in the Etudes sur les Schistes Crystallins published by the International Geological Con- gress, which met in London, in September, 1888. It would not be possible, were I to attempt it, to express my indebtedness to Professor Hermann Credner of Leipzig, whose masterly and philosophical work, Elemente der Geologie, as it stands in the sixth edition, 1887, is still without a rival in our language, as a storehouse of geological facts and principles. It is some gratification to me to find my own conclusions on some of the more important points in connexion with the genesis of the crystalline rocks so thoroughly in accord with those of Thomas Macfarlane, Esq., F.E.S.C., the result in his case of very extensive experience both as a metallurgist and as a field-geologist...