Vol. 59.] MAGNETITE-MINES NEAK COGNE. 



63 



The Rev. E. Hill gave particulars of the difficulty experienced 

 in getting the ore down. So far as he had seen, the ore passed into 

 the serpentine : no junction or divisional line was visible. At the 

 Filon Larsine the serpentine crossed the valley as a dyke. He 

 thought that it was hardly necessary to suppose the ore carried up 

 as a solid body : a viscous material might vary much in composition 

 through its mass. 



Prof. Sollas remarked that the explanation of the Author seemed 

 to avoid some of the greater difficulties of Vogt's hypothesis. It 

 was not easy to concede the migration of magnetite-molecules 

 through long horizontal distances in an already ejected and con- 

 solidating magma, and a differentiation prior to eruption appeared 

 more likely. The phenomena of Ovifak were more obscure : it was 

 hard to conceive of masses of iron with a specific gravity of 7, some 

 of them weighing 25 tons, being floated up by liquid basalt of 

 specific gravity about 3, unless the molten rock possessed a greater 

 velocity than could be fairly attributed to it. There was, besides, 

 much evidence to suggest that this iron had resulted from the 

 reducing action of carbon upon the basalt, for the latter penetrated 

 carboniferous shales, and in analogous instances in Greenland was 

 found in association with graphite and spinel. 



Prof. Watts called attention to the paper of Sir Archibald Geikie 

 & Mr. Teall on the banded gabbros of Skye, in which it was shown 

 that in some cases the iron-ores remained liquid until the other 

 rock-constituents had consolidated. Under these circumstances it 

 was possible that iron-ores might be injected as dykes, in the last 

 stages of consolidation of an igneous magma. 



The Authoe said that he had not attempted to lay down a general 



theory of iron-ores, but although he was aware that many were 



of sedimentary origin, the field-evidence which he had described 



rendered this in his opinion absolutely impossible. Moreover, though 



he was very well acquainted with the calc-mica-schists, lie had never 



seen anything like a bed of iron-ore in them. He regarded the 



suggestion advanced by the President as in the highest degree 



improbable. The point, to which from the first his attention had 



been directed, was whether the magnetite passed into the serpentine 



or not. He could not resist the evidence in favour of the former. 



In reply to Mr. Hill and other speakers, he had in the paper pointed 



out that the magnetite must have been nearly solid, though apparently 



it was partly melted at the exterior ; but the point which had 



impressed itself on his mind was that it was carried along like a 



solid. He thought that the difficulty raised by Prof. Sollas was 



not really serious, for the peridotite-magma probably was not in 



a truly fluid but a viscous condition, so that the heavier mass 



could be moved on as a stone in mud by the lighter. In reply to 



Prof. "Watts, he said that cases of gradual melting down of one rock 



by another were well known, and though the melting-point of olivine 



was apparently below that of magnetite, that of other minerals 



might be higher. 



