Among the metamorphic phenomena which seem to me deserving of more attention than they have yet received, I have been especially interested by those existing in the brecciate formations. They are, of course, in the main, two-fold; namely, the changes of fragmentary or rolled-pebble deposits into solid rocks, and of solid rocks, vice versa, into brecciate or gravel-like conditions. It is certainly difficult, in some cases, to discern by which of these processes a given breccia has been produced; and it is difficult, in many cases, to explain how certain conditions ofbreccia can have been produced either way. Even the pudding-stones of simplest aspect (as the common Molasse-nagelfluhe ofnorth Switzerland) present most singular conditions of cleavage and secretion, under metamorphic action; the more altered transitional breccias, such as those of Valorsine, conceal their modes of change in a deep obscurity: but the greatest mystery of all attaches to the alterations of massive limestone which have produced the brecciated, or apparently brecciated, marbles: and to the parallel changes, on a smaller scale, exhibited by brecciated agate and flint.