PLUTOXIC ROCKS OF GA.RABAL HILL AND MEALL BREAC. 119 



sequence agrees ia the main with that suggested by the facts 

 described in this communication. 



A most interesting extension of the same principle has been 

 recently made by Prof. Vogt of Christiania.^ This observer shows 

 that ore-deposits similar to those of Ekersund in JN'orway and 

 Taberg in Sweden must be regarded as the result of concentration 

 in strongly basic magmas. He discusses the various known causes 

 which may operate to produce such concentration, and inclines to 

 the view that the most important are those which act independently 

 of the separation of constituents in the form of crystals or crystalline 

 aggregates. He recognizes the existence of three such causes : — 



(1) Soret's principle, which, as he points out, is a necessary 

 consequence of Van 't Hoff's theorem relating to osmotic pressure."" 



(2) The principle of Gouy and Chaperon.^ 



(3) Magnetic attraction. 



The last cause he considers can come into operation only after 

 concentration has been started in some other way. It may then 

 produce an increase in the amount of concentration and an extension 

 of the space in which such concentration is taking place. 



Prof. Vogt, therefore, regards the formation of actual magmas 

 having the composition of the ores as a possibility. This view was 

 not contemplated by us ; but the facts described by him lend great 

 support to it. Thus the Ekersund deposits occur as dyke-like 

 masses in a variety" of norite (labradorite-rock) extremely poor in 

 ores and ferro-magnesian silicates. 



As illustrating the differentiation which may be produced in a 

 magma by diffusion -processes, Prof. Yogt describes a dyke near Huk 

 in the Christiania district. This dyke is an acid orthoclase-porphyry 

 in the centre and a somewhat basic kersantite at the margins. The 

 transition from one type of rock to the other is perfectly gradual, 

 and seeing that the margins represent the parts which must have 

 been kept cool by the passage of heat into the surrounding rock, it 

 seems reasonable to attribute the concentration of basic minerals at 

 the margin to the operation of Soret's principle. In the plutonic 

 area described in this communication the basic rocks are found at 

 the margin, but they do not form a continuous zone. 



Another case of the association of ores with eruptive rocks, in 

 such a way as to suggest that the former results from segregation 

 processes taking place in the latter, has been described by Mr. Orville 

 A. Derby." 



In conclusion, we may call attention to a case which bears a very 

 close resemblance to that which we have described. On the east 



^ ' Om dannelsen af de yigtigste i Norge og Sverige representerede grnpper 

 af jernmalmforekoraster,' Greologiska Koreniiig i Stockholm, Forhandl. Band 

 xiii. (1891) p. 476. [See Abstract in Geol. Mag. for 1892, p. 82.] 



2 ' Die Rolle des osmotischen Druckes in der Analogic zwischen Losungen 

 und Gasen,' Zeitschr. f. phys. Chemie, Band i. (1887) p. 481. 



3 ' Sur la concentration des dissolutions j)ar la pesanteur,' Ann. de Chim. 

 et de Phys., ser. 6, tome xii. (1887) p. 387. 



* * On the Magnetite Ore Districts of Jacupiranga and Ipanenia, Sao Paulo, 

 Brazil,' Amer. Journ. Sci. vol. xli. (1891) p. 311, 



Q. J.G.S. iS^o. 190. K 



