In exercising a debater's right to reply to criticism, I shall be extremely brief. My contentions, in two essays on the above subject,1 were as follows:—

1. That the classification of rocks by silica percentage is unsatis-factory, and that a more natural basis for classification is available in the degree of saturation of the constituent minerals.

My critic, Mr. Scott, has not expressed any opinion on this pointof comparison.

2. That the field evidence is overwhelmingly favourable to the separation of the rock minerals into two groups, those of one group being stable in the presence of free silica under magmatic conditions, those of the other group unstable; and that such experimental work as has been done supports that separation.

The sudden change of strike of the Chalk in South-East Suffolk, exceeded only in abruptness at ,the western limit of the London Basin, is accompanied by Tertiary phenomena equally interesting, and the occurrence of the Palæozoic platform at a small depth under the Chalk in the deep borings at Culford, Stutton, Harwich, and Weeley is, in this connexion, very significant.

Two years ago, in discussing the thermal energy of the earth, I suggested that while it had become impossible to deduce the earth's age from its thermal condition alone, Kelvin's problem might profitably be reversed by accepting the earth's age as a known factor, and deducing with its help the thermal history of the earth. This paper is a first attempt to attack the new problem then suggested. For geological purposes, one of the most fundamental aspects of the problem is that relating to the depth within the earth's crust at which temperatures are attained such that, under suitable conditions of pressure, molten rock magmas may exist. The determination of the minimum depth of possible rock fusion is a first essential to any adequate theory of vulcanism, and indeed of igneous activity in general. It is not sufficient, however, to ascertain that depth for present conditions alone, its variation during the earth's geological history must also be investigated ; for if, as is generally believed, the earth is a cooling body, the depth must be slowly increasing, and in former periods it must necessarily have been nearer the surface than it is now. In the limiting conditions both of position and time, tlie depth of fusion may have been at, or so near as to be for all practical purposes at, the surface itself. That is to say, at the beginning of geological history the earth may have been in a molten condition at, or immediately below, the then existing surface

So far, the source of the heat lost from the earth has been assigned only to the disintegration of the radio-active elements. Besides this, heat is liberated near the surface by the processes of weathering. The decomposition of one gram of average rock is accompanied by the generation of about 120 calories. Owing to its extreme slowness, this source of heat generation is of practically no importance except locally, where for a time it may succeed in appreciably steepening the temperature gradient. The heat liberated by weathering is probably balanced by the absorption of heat at greater depths, which is involved in the processes of metamorphism. Of much greater importance is the question whether the earth can still be regarded as possessing an ancient heritage of heat dependent on its origin.

The recognition of teschenites in the Midland Valley of Scotland, first by Teall and later by several other petrographers, has led to the discovery of numerous varieties of analcite-rich rocks in th same area. One of the most interesting of these is the unique rock discovered in an old quarry at Barshaw near Paisley, during the remapping of the district by the officers of the Geological Survey of Scotland. A preliminary description of this rock as a weathered theralite was given by Mr. E. B. Bailey in 1909. In the previous year a chemical analysis by Mr. E. G. Radley had appeared. A fuller description is given in the Survey Memoir on the Glasgow District (1911). According to Mr. R. G. Carruthers the theralite is probably part of an igneous mass which is persistently found at the Hurlet Limestone horizon of the Carboniferous Limestone Series in the Paisley district. Tlie petrography of the rock, as investigated by Mr. Bailey, makes clear its relationship to the melanoeratic ijolites of Madagascar described by Lacroix, and named ‘bekinkinite’ by Rosenbusch. It is considered as a special modification of the Paisley teschenite or dolerite.

Although numerous papers, principally by the late E. Wilson, describing the beds of the Rhætic and the Lower Lias of Notts, were published while exposures existed along railway cuttings in the south-east of the county, no account of the precise positions of important fossils is to be found. These exposures are now all grass-grown, but the junction of Rhætic and Lias may be seen in a number of quarries where hydraulic limestone is worked, either for cement or for road material. One of the best is at Owthorpe. Those at Cotgrave Gorse, Barnstone, Plumtree Wolds, and Normanton Hills are confirmatory. The layers rich in Foraminifera have been of value in correlating these sections.

The last Study brought out many resemblances between Edrioasteroidea and Asteroidea, much closer than have been recognized even by some who have suggested a derivation of the latter Class from the former. In the present Study it is proposed to inquire whether that hypothesis, or any similar hypothesis, is tenable.

This Plesiosaur was collected from the Oxford Clay at Peterborough by A. N. Leeds, Esq., F.G.S., and was acquired for the Hunterian Museum, Glasgow University, by Professor J. W. Gregory. The specimen has many striking resemblances to Cryptocleidus oxoniensis, but detailed examination showed that it could not belong to that species, and the differences existing in the paddle, shoulder-girdle, number of vertebræ, and conditions of ossification are such that it cannot be retained in that genus as now defined. The paddle closely resembles that of Tricleidus, but the shoulder-girdle alone is sufficient to prevent the inclusion of the specimen in that genus. It seems advisable to create a new genus, and as it may be some time beforea full description can be published a preliminary account is here given.

The facts concerning the Curvature of the Subvective Grooves have been summarized in Study IV(1914, p. 122). They show that no distinction can be drawn in this respect between. Edrioasteroidea and Cystidea Diploporita, as some have pretended.

A description on modern lines of the Brighton chalk below the (old) zone of Actinocamax quadratus is to be found in Dr. Rowe's “Coast Sections”, pt. i, p. 346. The principal points that he makes are that between Ovingdean Pumping Station and Black Rock the cliffs display below the (old) zone of A. quadratus a thickness of 58 feet of chalk, the whole of which, he assigns to his zone of Marsupites, and further to his Marsupites band as Uintacrinus was not found in it, but he notes that Uintacrinus has been found on the reefs. These points have, as far as I know, stood unchallenged hitherto, but I am unable to reconcile them with my experience.

In 1890 Vine described Membranipora gaultina a uniserial Cheilostome from the Cambridge Gault. A peculiarity of this species was ‘a puckering’ or ‘folding-in of the wall below the area’ shown by Vine in his figure of the type-specimen. Unfortunately this specimen cannot be found; but a well-preserved paratype specimen (from the same horizon and locality) exhibits the same structure and shows that this is due to the collapse of the extra-terminal front wall along a median line proximal to the aperture. Other zoœcia of the same specimen show the extra-terminal front wall entire and a thin median seam—or rhamma—running proximally from the aperture. The rhamma appears in some instances as a ridge and in others as a depression; in some zoœcia it is not apparent; and when visible is extremely fine. In many cases the front wall is broken along it (as in Vine's figure) and thus the rhamma probably indicates a line of weakness. It is suggested that the rhamma is a ridge bounded by two furrows and that the front wall is thicker at the ridge and thinner in the furrows. The rhamma, then, would easily be broken off, and in such specimens a depression would appear in its place; and a thin groove on each side would constitute a general line of weakness which would account for the frequent breaking down of the front wall.

Charnwood Forest, since 1891, the date of the last paper by Canon E. Hill and myself, has been investigated by the Geological Survey. Though part of their map and the accompanying memoir have not yet been published, the general results of their work have been announced by Professor W. W. Watts, by whom most of it was executed. As we stated at the time, we were far from being satisfied with some important points in our own conclusions; so that since my return to Cambridge I have studied my specimens and slices from the north-western region, which had presented to us the more serious difficulties. In 1891 I had been led to regard the characteristic rocks of Peldar Tor and High Sharpley as lava-flows, but considered the dominant rocks of Bardon Hill to be mainly pyroclastic. Professor Watts, however, maintained the intrusive character of the first and second, while taking the same view as myself about the third. The lava-flow hypothesis had appeared to me the more probable, because I doubted whether a mass so large as the Peldar-Bardon porphyroid, if intrusive, could have maintained throughout a texture so uniformly fine-grained, and I had found in the Bardon quarries fragments of it embedded in rock which I then supposed to be a somewhat altered tuff, closely related to the High Sharpley lava.

It is well known that in various regions of Europe, Asia, and Australia dark pieces of glass are found which, in view of their peculiar shape and sculpturing, have attracted the attention of observers for a good many years. The name of obsidianile was given to these objects by R. H. Walcott. It expresses their resemblance in petrographic nature to obsidian.

In the description of the skeleton of Metriorhynchus given in vol. ii of the Catalogue of the Marine Reptiles of the Oxford Clay, very little could be said about the fore-limb. The only partsthen known were a number of humeri and, in one or two cases, somesmall disc-like bones which were regarded as the radius and ulna:the most complete fore-limb was figured on p. 172 of the volumequoted. In order in some degree to fill this gap a short account wasgiven (torn, cit., Introduction, p. xiii) of an imperfect fore-paddle ofa specimen of the closely related Geosaurus graeilis from the Solenhofenlimestone. This beautiful skeleton, originally described by V. Ammon(Geognostische Jahreshefte, vol. xviii, p. 12), has recently beenacquired by the British Museum, and a photograph of it is given asa frontispiece to vol. ii of the Catalogue of Oxford Clay Reptiles. The specimen is beautifully preserved, and is remarkable for showingthe outline of the tail-fin and other traces of the soft parts.

In descriptions of Recent and Tertiary Terebratulids at least, the type of folding is often hardly mentioned, and is left to be inferred from the figure, in spite of the fact that Douvillé in 1879 showed that it might be even of generic importance. Buckman has more recently drawn attention once more to the importance of this character and has shown that its due consideration may lead to fruitful results in classification. The object of this paper is to point out further cases in which it has been neglected.

Scattered throughout the Palæozoic rocks there are frequently found certain fossils, in most cases represented by single detached plates, which have been ascribed by various authors to the sub-class Cirripedia. Thus we have fossils described under the names Balanus, Cirripodites, Lepidocoleus, Plumulites, Pollicipes, Protobalanus, Scalpellum, Stenotheca, Strobilepas, and Turrilepas, but in the case of Balanus, Pollicipes, and Sealpellum, the reference to those recent genera is quite unsupported by the structure of the fossils.

Up to the present marine fossils have been recorded in Middle Coal-measures in the South Lancashire Coal-field at two horizons only. They are (1) in the banks of the Tame, near Ashtonunder-Lyne, found by Professor A. H. Green, and at Ashton Moss Colliery, about 750 feet above the Great Mine, discovered by the late George Wild. (2) Mr. H. Bolton, F.R.S.E., informs me that the Californian or Thin Bed of Fulledge Colliery, Burnley, which is 410 feet above the Arley Mine, is a marine horizon.

The subject of this paper is a group of three intrusive masses of igneous rock, possibly laccolitic in their origin, whose outcrops are situated within a radius of a mile from the village of Llanfairfechan, on the north coast of Carnarvonshire.

In the preceding paragraphs it has been shown that, after the inland ice had attained its southernmost limit and had spent its force, there commenced in Southern England the last of many stages of land depression. This carried with it a complete reversal: the temperature was raised, the periphery of the inland ice melted, its pressure was lessened, and a rapid rise of the land—the Mousterian elevation—introduced a great rise, to which the origin of the submerged forests bears witness. Nevertheless, the greatest part of the land depression persisted even after Mousterian time, and this explains the continuance of the melting and its increased rapidity, as well as the rapid northward withdrawal of the inland ice, which we shall soon consider.

This remarkable rock occurs as a marginal phase of the granulitic albite granites which are found between Noqui and kilometre 16 on the road to San Salvador. In the hand-specimen it shows a finegrained slightly foliated structure with a characteristic pepper and salt appearance, due to the sprinkling of lustrous black plates of riebeckite among iron-stained granular crystals of quartz and felspar. A few small porphyritic crystals of orthoclase are embedded in the main mass of the rock, making up approximately 15 per cent of the whole.