460 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 11, 



have been found in several places near Glasgow. In the whole of 

 the above localities, however, it is a comparatively rare phenomenon, 

 and might be accounted for by the reduction of dichloride of copper 

 sublimed from a volcanic vent — a process of which we have indica- 

 tions in the occurrence of oxychloride of copper in the lavas of 

 Vesuvius. It is difficult, however, to consider this an adequate 

 cause for the metallization of a mass of rocks which, from their 

 appearance on Isle Eoyale, Michipecoton, and other points on the 

 northern and eastern shores of the lake, must cover an area of many 

 thousands of square miles. Another hypothesis, suggested by Miiller, 

 supposes the copper to have formed part of the felspathic component 

 of the trap rock when in the unaltered state — a view that is sup- 

 ported by the occasional occurrence of protoxide of copper in small 

 quantities, usually less than 1 per cent., in several anhydrous silicates, 

 such as felspar, orthoclase from Schemnitz, and Amazon-stone from 

 Siberia, epidote from S. Marcel in Piedmont, idocrase in the Nor- 

 wegian variety called cyprine, and olivine*. This hypothesis may 

 be modified by supposing the copper to have existed in the trap as 

 sulphide, mechanically interspersed in minute quantities, in the same 

 way that it is found in the coarse metal slags of copper-furnaces. On 

 the other hand, we have the recorded statement of Whitney f, who, 

 in an analysis of the trap from Isle Hoy ale, found it to contain no 

 copper, although it was specially sought for ; but traces were found 

 in the sandstones overlying the trappean series. This piece of nega- 

 tive evidence, however, must not be overestimated, as it would in 

 any case be very difficult, if not impossible, to determine the original 

 composition of the traps as they exist at present. They have un- 

 doubtedly undergone considerable change, as is shown by the presence 

 of hydrated minerals, such as chlorite and zeolites, as well as calcspar. 

 Rammelsberg has found as much as 0-56 per cent, of protoxide of 

 copper in a lava from Vesuvius from the eruption of 1811 — a rock 

 not very dissimilar in composition from the Lake Superior trap ; and 

 although we have great masses of copper concentrated at single 

 points, it must be remembered that the percentage contents of the 

 deposits selected as rich enough for working are included between the 

 narrow limits of one-half and two per cent. ; and supposing the con- 

 tents of these deposits to be uniformly distributed through the whole 

 mass of the traps, the slate of division would be so great as to render 

 the copper difficult of detection. The presence of copper in the sand- 

 stones suggests another origin — namely, that it may have originally 

 been deposited with the quartz-ore sediment as a finely divided sul- 

 phide from sea- water under the influence of organic matter, and by 

 subsequent oxidation and solution have been removed and collected in 

 the rocks below. Cupriferous sandstones and other sedimentary rocks 

 are comparatively common, as, for example, the Carboniferous Sand- 

 stones of Nova Scotia and the Kupfer Schiefer, which contain sul- 

 phides of copper in an unaltered form, partly on account of the state 

 of aggTCgation of the deposited mineral, and partly from the texture 



* Bischof, Geol. vol. ii. pt. 3. p. 189. 

 t Q-eology of Lake Superior, pt. 2. p. 87. 



