An average analysis is made of an altered intrusive greenstone occurring in the zone of the Kiruna greenstones with the special object of proving whether any local change of chemical composition is occasioned during the alteration processes by the uneven distribution of epidote, as was earlier supposed by the writer. The analysis confirms this, but it also shows that chemical inhomogenities must originally have occurred in the rock-mass. The relations found thus do not indicate great local changes of the chemical composition during the mineral alteration processes still less a change of the bulk composition of the rocks.

A comparison of a number of analyses of spilites and related An-richer rocks affords the following facts: among them there occurs a small group of An-rich members, which are designated here as diabases. Most of the analyses refer to rocks richer in albite, which are here called spilites in the proper sense. In the felspar diagram the projection points of the calculated felspar mixtures of spacing the rocks form a narrow zone close to the plagioclase line extending from 7 per cent An to 58 per cent An. As the dividing point between the diabases and the spilitic members a mixture with about 40 per cent An is accepted. When adding to the diagram the keratophyric rocks associated with the spilites, the whole distance from about 60 per cent of An to the immediate vicinity of the Ab-corner would be filled up.

Between the spilites and other rocks of equal femic composition and of about equal SiO2 content there exists in the felspar diagram an area in which very few rocks are represented. Petrographically the spilites are thus distinguished as a special rock-type. The boundary towards the ordinary basalts and diabases at the An-rich end of the series on account of the relations of the felspars alone is more dubious, as the individual basalts or diabases may be very poor in potash, but a difference seems to exist here also.

Other chemical qualities characteristic of the spilites are deficiency in alumina and high content of ferrous iron and of TiO2. The low percentage of Al2O3 is the main cause of the soda-rich composition of the felspar, as the CaO-content of the rocks is not low. The An-poor composition of the felspar in the rocks, furthermore, is accentuated by the entrance of An-molecules into the pyroxene during the crystallization of the magma, the generation of olivine being thereby prevented. This mineral, or pseudomorphs of it, is seldom recorded from the spilites. The high content of FeO is distinctive, not only for the spilites proper but also for the diabase members truly belonging to the series.

The importance of the high content of FeO for the course of crystallization in the An-poor members of the spilites is discussed. The consequence is found to be a change of the eutectic ratio of pyroxene and plagioclase contained in the concentration-diagram of plagioclase and pure MgO-pyroxene. The eutectic line for pyroxene and plagioclase during the crystallization of the magma will thus be reached earlier than shown by the diagram. In this way the textural relations of the rocks are explained without the hypothesis of autometamorphic changes and of an earlier An-rich plagioclase. In the appearance and mineral composition of the rock there is nothing compelling an assumption of this kind, unless the alteration of the minerals to compounds richer in water and carbon dioxide is referred to the period of consolidation. This would imply the fixing in the consolidated rock of these elements, but not a change of the other oxides. As to the time of the generation of the secondary minerals we do not know anything with certainty. The fact that the spilite group includes members which are poor in water and carbon dioxide, or are not decomposed at all, shows that a high content of these compounds is not necessarily inherent in the spilitic magma.

The solid and liquid states are defined on the basis of the viscosity-temperature curve.

Glasses are amorphous solids. Their state of molecular aggregation is fundamentally different from that of liquids: they are not super-cooled liquids.

It is suggested that porphyritic texture may result from an even rate of magmatic cooling on account of the abnormal increase in viscosity which occurs before vitrifaction.

In the first place let us deal with the origin of the word. It was first used in 1807 by F. Buchanan in A Journey from Madras through Mysore, Canara, and Malabar. In dealing with the iron-ore seen during his journey he wrote:—“In all the hills of the country the ore is found forming beds, veins, or detached masses in the stratum of indurated clay, that is to be afterwards described, and of which the greater part of the hills of Malabar consists. … What I have called indurated clay … is one of the most valuable materials for building. It is diffused in immense masses, without any appearance of stratification, and is placed over the granite which forms the basis of Malayala. It is full of cavities and pores, and contains a very large quantity of iron in the form of red and yellow ochres. In the mass, while excluded from the air, it is so soft, that any iron instrument readily cuts it, and is dug up in square masses with a pickaxe, and immediately cut into the shape wanted with a trowel, or large knife. It very soon after becomes as hard as a brick, and resists the air and water much better than any bricks I have seen in India. … As it is usually cut into the form of bricks for building, in several of the native dialects it is called the brick stone. … The most proper English name would be Laterite, from Lateritis, the appellation that may be given to it in science.”

In July of 1929 I visited the classic locality of Piltdown under the guidance of Sir A. Smith Woodward. After paying homage at the tomb of Eoanthropus we descended from the plateau to the low-level gravel terrace that flanks the flood-plain of the River Ouse. At a place known locally as Sharp's Bridge (marked on the accompanying map by a cross) there is a new but already extensive gravel pit. A spot-level on the map near the pit gives the approximate level of the top of the gravel terrace as 38 o.d.; the face of the pit is dug to a depth of from 8 to 10 feet below this level, and seems to reach down to about the level of saturation determined by the water of the river.

Owing to absence abroad Dr. J. Parkinson was unable to see a proof of his letter in the December number on the Deinosaur beds of Tendaguru; the accidental omission of a word spoilt the meaning of one sentence. On p. 560, lines 20–1 should read: “brackish water conditions alternated with local marine episodes.”