March 20, 1890] 



NATURE 



465 



Up to this point, the new explanation of the origin of drift 

 gold seems feasible, and, indeed, almost conclusive. The gold 

 is present in minute quantities in the water of the drift, and this 

 fact has been conclusively demonstrated experimentally by 

 various investigators, among whom may be mentioned Messrs. 

 Newberry and Skey. But it is one thing to prove that chloride 

 of gold exists in the drift waters, and quite another thing to sug- 

 gest in what manner and by what agency the precious metal has 

 been reduced from its salt, and deposited in the form of coarse 

 or fine grains or in that of large and strangely-shaped nuggets. 

 Precipitation was the first and most obvious suggestion. The 

 addition, for instance, of a minute quantity of sulphjjite of iron to 

 a solution of chloride of gold would cause the formation of 

 minute particles of metallic gold, and sulphate of iron, of course, 

 is present in Nature abundantly. But such an explanation 

 would only account for the formation of the very finest gold 

 dust. It would give no solution of the origin of coarse gold 

 and nuggets, nor would it account for any of the many peculiar 

 anomalies of which I shall presently mention some striking 

 examples. 



In order to afford a possible extension of this purely chemical 

 theory which might give a clue to the origin of nuggetty gold, it 

 has been pointed out that if a crystal of some sulphide, such as 

 iron pyrites, be immersed in a solution of chloride of gold, it will 

 be covered with a film of metallic gold. Following the track 

 of investigation thus apparently opened up, it has been in- 

 geniously suggested that possibly the material of the metallic sul- 

 phide, and that of the golden film, may be regarded as a sort of 

 miniature electric battery, in which the gold would form one 

 anode and the pyrites the other. A current would pass between 

 the two, and the result would be the deposition of metallic 

 gold upon the film, at the same time that the material of the 

 pyrites would continually become decomposed. The electro- 

 plater, in his laboratory, places the salt of gold in his bath, and 

 uses an ordinary battery from which to obtain a current suffi- 

 ciently strong to deposit gold upon the articles to be plated. 

 But in this case it was suggested that the article to be plated, 

 which was the film of gold itself, might be regarded as one of 

 the elements supplying the current. The theory seems from the 

 outset somewhat far-fetched, and it is open to very strong objec- 

 tions on the ground of improbability. The amount of material 

 which the electroplater has to use up in order to deposit an ounce 

 of gold is very considerable, even in the most efficient forms of 

 batteries known to science. It is scarcely conceivable that a 

 piece of pyrites, weighing about two pennyweights, would, by 

 its decomposition, afford sufficient current to deposit an ounce of 

 gold. Yet something of the sort would have to be established 

 before it could be proved that electro-chemical action in situ 

 supplies the electric current as a reducing agent. 



In seeking for an explanation of the deposition of gold which 

 would affiDrd a surer or more probable basis for conjecture, I 

 was at first mainly influenced by two remarkable facts which 

 could hardly be referred to any imaginable phenomena of a 

 chemical or electro -chemical origin. These were that in a drift 

 supplying gold in abundance it is by no means uncommon to 

 find a patch in which the gold gives out altogether, and is picked 

 up further along the line ; and the second was that there has 

 always been observed at many of the leading goldfields a certain 

 correspondence between the richness of the alluvial drifts and 

 reefs and the points of the compass. The direction in which 

 the richest drifts run may vary from one locality to another. But 

 no matter how broken in contour the country may be, there is 

 almost always a marked parallelism between the richest drifts. 



Taking these and one or two other facts as a starting-point, 

 I was led to form the hypothesis that the probable origin of the 

 deposition of gold is to be found in thermo-electric earth-currents, 

 probably generated by the unequal heating of the surface of the 

 earth by the sun's rays in passing from east to west. This theory 

 of earth-currents has attracted a good deal of attention in Aus- 

 tralia, and it is remarkable how rapidly facts in support of it 

 have been brought forward during the past few months. It 

 would be impossible for me, within brief limits, to refer to all of 

 these ; but it will be of interest to summarize a few of the 

 leading points : — 



( i) The existence of earth-currents has been frequently demon- 

 strated, and has attracted special attention since the invention 

 of the telephone. In 1880, Prof. Trowbridge, of Harvard, con- 

 ducted a series of experiments at the Observatory, and recorded 

 it as one of his results that these currents appeared to be most 

 pronounced along the water-courses. 



(2) In Victoria remarkable instances of deflection of the com- 

 pass have been particularly numerous, hinting at the presence of 

 strong currents, more especially at the lines of junction between 

 permeable and impermeable rocks. 



(3) There is a remarkable relation between the conductivity 

 of the adjacent rock country and the richness of an alluvial drift. 

 Thus, in passing through slate or below an overhanging mass of 

 basalt, the drift is generally richer than in passing through moist 

 sandstone, suggesting that, where an earth-current is concentrated 

 along the line of the water in consequence of the presence of 

 rocks of low conductivity, the process of deposition has been 

 facilitated. 



(4) There are places at which the gold gives out altogether, 

 although no discernible change has taken place in the nature of 

 the country. These places seem to be the localities of a sort of 

 short-circuiting, which we may readily suppose to take place 

 very frequently in earth-currents. 



(5) At particular pinched localities the current would be 

 peculiarly strong, and would lead to the formation of nodules or 

 nuggets of gold, the existence of which cannot be satisfactorily 

 explained by any chemical theory hitherto advanced. 



(6) Nuggets of an alloy of gold and copper have sometimes 

 been met with, and the two metals have even been found to lie 

 in alternate layers, suggesting that at one time a copper salt, 

 and at another a gold salt, has been subjected to the action of a 

 reducing current. 



(7) In presence of a large amount of organic matter, it is 

 almost invariably found that a drift becomes especially rich. 

 The formation of acid by decomposition is what would be 

 peculiarly required to facilitate the passage of an earth-current 

 through the water of an underground drift, the existence of 

 free acid being the requirement for an artificial electro-depositing 

 bath. 



(8) Conversely, the vicinity of large masses of calcite has been 

 observed to be most inimical to the richness of a drift, and, of 

 course, this could be explained by the fact that the carbonate of 

 lime would destroy the free acid, and reduce the conductivity 

 of the water so as to impede the transmission of a current. 



(9) The peculiar shapes of the grains of what is known as 

 coarse gold, are very suggestive of the action of a feeble current 

 in piling up the metal upon the prominent portions, and leaving 

 deep indentations between. Electric action of an extraneous 

 nature is also strongly indicated by the strange strings and fila- 

 ments which are constantly being met with. 



(10) If we accept the crenitic theory of the origin of quartz 

 reefs, the theory of earth-currents would at once apply with 

 particular force to show how the action of such currents in hot 

 siliceous solutions would produce a formation of gold simultaneous 

 with that of quartz, thus accounting for the finely divided state 

 of the gold in such reefs. 



(11) At the same time it is necessary to account for the exist- 

 ence of the large masses of gold which are sometimes found 

 associated with quartz, at places where the reefs become narrow 

 in pinched localities. The theory of precipitation cannot account 

 for these. But that of earth-currents would naturally lead us to 

 expect the phenomenon, because in such a locality, while the 

 formation of quartz would be retarded, the formation of gold 

 would be accelerated by the concentration of the current as 

 already explained. 



The hypothesis is thus well supported hyprimd facie evidence. 

 For the experimental detection of earth-currents on goldfields I 

 have strongly recommended the close observation of the most 

 minute deflections of the magnetic needle, especially in under- 

 ground workings. I believe also that the use of the telephone, 

 as in Prof. Trowbridge's experiments, will be of great service in 

 indicating the lines of greatest conductivity in the earth's crust, 

 and in enabling us to decide whether these are identical in gold- 

 fields with those lines in which the drifts contain the richest 

 gold. George Sutherland. 



Angas Street, Adelaide, South Australia. 



THE PRIMITIVE TYPES OF MAMMALIAN 

 MOLARS. 



SO much light has recently been thrown on the origin 

 and mutual relations of the Mammalia by the 

 labours of the Transatlantic palaeontologists, that in the 

 case of the limbs we have long since been able to trace 

 the evolution of the specialized foot of the Horse from 



