A review of the literature, and our own observations have led us to the conclusion that granitization, widely defined as the process by which solid rocks are converted to rocks of granitic character, has been an important process in the evolution of many subjacent masses of granite and allied rocks, such as granodiorite and diorite, in erogenic regions.

Granitization of pre-existing rocks seems to involve two processes: (a) Metasomatism under the influence of permeating highly energized fluids—emanations—ahead of advancing magma; and (b) Mechanical penetration by magma. During the first process metasomatic exchanges take place between the minerals of the country-rock and its pore fluid: the reactions tend towards the achievement of equilibrium under the existing physico-chemical conditions. This metasomatism is selective, and promotes the convergence, in bulk and mineral composition, of rocks which were originally different, towards a common end-product. During the early stages conditions are essentially those of thermal metamorphism, but with rising energy level and increasing magmatic contributions the final conditions are dominantly “igneous”.

The mechanical penetration of the altered rocks and their mixing with magma may follow the first process without a break. Alternatively, intrusion may occur before metasomatism is completed, or has even had appreciable effect. The mixing or intrusion stage is accompanied by reactions, which tend towards the attainment of equilibrium, under magmatic conditions, between the magma and the foreign material. In terms of bulk composition the products will be intermediate between that of the magma and that of the incorporated material.

The metasomatism trends of the various examples that we have discussed are plotted together in Text-fig. 8. The diagram brings out the convergence of the curves from the normal sedimentary field to the rhyolite-diorite line, which is itself the locus of mixtures between metasomatized products and acid parental magmas.

Granitic rocks may originate as the result of either or both these processes. That which predominates in any particular case depends on the supply of emanations and the local physico-chemical conditions. In the deeper parts of the granitic layer complete or selective fusion is to be expected, and the magmas so formed may rise and granitize the overlying rocks: firstly, by metasomatism, and at higher levels chiefly by mechanical penetration. At still higher levels, owing to loss of energy, rising magma is unable to cause extensive modification of the pre-existing rocks, and forms true intrusions.

The fertile highlands of Kenya, of which the great mountain of that name is the northern outpost, are divided from those of Abyssinia by a great desert nearly 400 miles wide. This tract is a gneiss peneplain of great age, whose degradation, without any regional interruption, has continued since Palaeozoic times. At the present date its undulations are scarcely noticeable, and its ill-supplied rivers do not often reach the coast. Such is the Uaso Nyero, which, rising from the north-western slopes of Mount Kenya, and running spasmodically for 200 miles eastwards through this desert, finally dies away in the great swamp called Lorian, which shifts its position up or down the valley in accordance with the seasons. The Tana River reaches the sea, and its valley also has the appearance of great antiquity; the long slopes down to the valley from its sides are hardly perceptible, and for two-thirds of its 400 mile course the river meanders sluggishly from one side of its valley to the other.

It will have been observed in our consideration of the physiography of the north-eastern corner of the Northern Frontier Province of Kenya Colony that there is strong evidence in support of the existence of fault scarps, which date their first uprising from the Jurassic age. The evidence in the Coast Province in the neighbourhood of Mombasa is not so clear, but if we examine this in the light of that from the northern frontier, it will be seen that there is a strong suspicion that the coast ranges, too, are horsts, dating their first rise from the Jurassic.

In the course of field work in North-East Yorkshire on the Dogger and associated rocks, in which we have now been engaged for some years, we have found several occurrences of a rock which we may call Black Oolite. In order to prevent any misunderstanding it will be well to say at once that this is not the magnetite-oolite of Rosedale, so often mentioned in the literature and no longer exposed, but quite a different type, which does not appear to have been described by any writer, though well worthy of special investigation. This is the more remarkable in that all the sections known to us where it occurs are referred to in nearly all the standard works. Incidentally we should like to remark that over and over again we have been amazed at the inadequacy and often the entire incorrectness of the petrographical descriptions given in the literature of numerous admirable exposures throughout the district. Not seldom we have found it impossible to reconcile with these descriptions our measurements and petrographical determinations of such sections, although there can be no doubt of their identity; it should also be remarked that sixty or seventy years ago many of these sections, especially in alum works, must have yielded even fresher and more easily determinable material than they do to-day.

As shown by his paper on “The Age of the Rocks and Topography of Middle Northern Iceland” (Geol. Mag., LXXV, 1938, 289–296), Dr. Hawkes realizes the great change of view concerning fundamental features of Icelandic geology necessitated by my discoveries in North Iceland of Pleistocene metamorphic glacial deposits. In fact, the changes involved seem to him so great that he finds the whole thing incredible, and he rejects what he terms my hypothesis. In this incredulity, I think, is to be found the reason for his being unable to repeat for himself some observations I made on the eastern side of Fnjóskadal. That I should have been mistaken is, I think, out of the question; a typical “striated pavement” is something a geologist of some experience cannot be mistaken about. In Skridugil I exposed a surface which quite definitely exhibited traces of glacial action. That “similar deposits” to those overlying the pavement “are found at various horizons in the older Tertiary Series of Iceland” (Hawkes, op. cit., p. 293) is unknown to me. The name “Skridugil” was given to me by a man who may not have understood to which locality I was referring.

This note is prompted by the recent discovery (by Mr. R. Casey) of ammonites of the jacobi zone at the base of the Folkestone Sands.1 These ammonites include A. jacobi itself and since, more-over, the jacobi zone is likely to be much quoted, it seems particularly desirable to inquire whether the name jacobi can be safely used.