work showed that the Santaveri trap of the Kadur District possessed a considerable resemblance to the Grey trap of Chitaldrug, so much so that it had almost been decided to class these two formations together and separate them from the Bellara trap. The radium determinations, however, show that the Grey trap and the Bellara trap have practically the same radioactivity (vide Nos. 23 and 24), which is about one third of that of any of the schists proper, while the Santaveri trap (No. 19) contains about three times as much radium and falls within the limits so far ascertained for the schists. This appears to confirm the original classification, which correlated the Grey trap with the Bellara trap, and which, consequently, has been allowed to stand. The case affords an illustration of the possible use of radium determinations as an aid to correlation of the highly metamorphosed members of the Archæan complex, amongst which it may often happen that the chemical and mineral composition, and the field relationships, do not afford sufficiently definite points of similarity or distinction. The value of such determinations will depend on the possibility of ascertaining fairly definite limits for the radium contents of the various rock groups or of the various members of such groups. Amongst the various gneisses and granites, which have Ieen divided into four great groups of distinctly different ages, it will be noted that the Charnockites stand apart from the others in virtue of their excessively low radioactivity, which is much lower even than that of the Dharwar schists. The Champion gneiss, Peninsular gneiss and the Closepet granite-which last is also a variable complex-all contain from 12 to 15 times as much radium as the Charnockites and four to five times as much as the Dharwar schists. The Charnockites have been shown by Holland to form a distinct petrographical province amongst the gneisses of Southern India and vary, as the result of magmatic segregation, from highly acid granites to norites and hypersthenites -all characterized by the presence of hypersthene and certain physical features-and the radium determinations fully confirm the distinct individuality of the parent magma. The varieties of Charnockite which have been examined show an increase of radium with increasing basicity, but the hypersthenite and quartz-magnetite ore-which are considered to be end products of the segregative process-show a relapse towards the mean value. The other gneisses and granites are very complex, and the determinations are not sufficiently numerous to permit of very definite conclusions. It may be noted that the members of the Champion gneiss series show least variation, those of the Peninsular gneiss rather more, and those of the Closepet granite the greatest variation. The auriferous quartz of the Kolar Field (No. 29) is interesting as falling into line with the members of the Champion gneiss series, with which it has been correlated on other grounds, and, as already pointed out, the altered schists (lode matter) in contact with, or in continuation of, the auriferous quartz veins show values intermediate between those of the normal schists and of the quartz. It is interesting also to note that the matrix of the Conglomerate (No. 27) has the same radioactivity as the clearly intrusive granite (No. 28), and this is in accordance with the view that the conglomerate is autoclastic and due to crushing of portions of the Champion gneiss series. The pegmatite cross-course (No. 39) is remarkable as yielding the highest result so far obtained. The pegmatite contains a large amount of tourmaline, and it was thought that this mineral might account for the high value. A small quantity of the tourmaline was separated and, though a definite determination was not made, the test was sufficient to show that it was not abnormally high, and that the high result of the rock as a whole was not due to this mineral. A single determination (No. 50) has been made of one of the very numerous dolerite dykes which are considered to be of Pre-Cambrian age but subsequent to the formation and folding of the Archæan complex. This rock is very similar in composition to the old hornblendic schists, which probably were originally diabasic flows and sills of a much earlier period. The result shows that the later rock contains more than twice as much radium as the earlier type, but no further inference can be drawn from a single observation. Summary. 1. These very ancient rocks, all of which are considered to be of igneous origin, contain remarkably little radium. 2. Amongst the various groups which have been differentiated on geological grounds, there are some striking differences in the radium contents of some of them. 3. In the case of a fairly uniform group of rocks (viz., the hornblendic schists of the Kolar Field) the radium content does not appear to vary with the depth from the surface. 4. Different igneous magmas appear to contain very different amounts of radium, and the latter, or the minerals which carry it, is subject to magmatic segregation. The amount of radium in the segregated portions of a magma sometimes increases and sometimes decreases with increase of basicity. 5. Amongst magmas, the more basic appear to be lower in radium than the more acid, and, in the products of granitic magmas, the pegmatites appear to carry more radium than the corresponding granites. The Charnockite magma, which was probably of intermediate composition, forms a striking exception, and is notable for its extremely low radioactivity. 6. In the case of rocks of somewhat similar character and composition, and in which other means of distinction or identification are lacking, a marked difference in the radium contents may afford a means of correlating them with known groups or formations for which the radium limits have been sufficiently determined. Bangalore, March 1917. TABLE I. Radioactivity of Rocks from the Mysore State. (The rock groups are arranged in order of age from the oldest to the youngest. The radium is given in units of 10-12 per gramme of rock.) gramme Group 1. Dharwar System-Lower (hornblendic) series. Serial Registered No. Deseription. Radium. Remarks. Group 2. Dharwar System-Upper (Chloritic) series. Santaveri trap, Bababudan 0-20 Chloritic trap, with some hornblende. Probably altered hornblendic trap. Group 3. Intrusives of Dharwar age-subsequent to the schists. Hornblende diabase, Baba- 0.16 Altered diabase intrusive into chloritic series. Chloritic trap, probably related to Bellara trap. Associated with ultrabasic intrusives. Champion Gneiss Series-intrusive into Dharwars. 0.05 0.07 0.05 40. Group 6. Charnockite Series-later than the Peninsular Gneiss. J2/824 Acid Charnockite, Chamraj 41. J2/822 Intermediate Charnockite, 0.10 XXIV. The Effect of Interionic Force in Electrolytes. D° PART I. In O the electrical forces which exist between the ions in an electrolytic solution produce an effect on the ionic mobilities? This question has often been asked but has never received a satisfactory answer, although it is of fundamental importance in the theory of electrolytic dissociation. Part II. of the following paper an attempt is made to provide an answer by establishing on the principles of the kinetic theory a general proposition on the effect of interionic forces. Briefly stated this is that whatever effect such. forces produce on the osmotic pressure of the free ions in an electrolyte in reducing it below what it would be were the forces nonexistent, they will produce the same reduction in the average velocity with which the ions move in carrying a current. The bearing of this result on the theory of the extent and character of the dissociation of strong electrolytes is then considered. In this part it is proposed to consider in this connexion the well-known difficulty in the theory of Arrhenius connected with the failure of the law of mass action for strong electrolytes. The failure can, I think, be shown to be of such a kind as to form an objection apparently insuperable, not only to the original *Communicated by the Author. |