Modes of Occurrence of Ores. 29 



is also observable at the contact of two different rocks, and which 

 is not unlike that of which we have spoken above. When the 

 mass of ore passes directly into the rock, oris intimately united 

 with it at the line of junction, this phenomenon is scarcely to 

 be expected ; when, however, this is not the case, there is- 

 frequently found, between the ore and the rock, a layer of 

 another substance, of a clayey or other composition, which, 

 as it were, incloses the mass of ore in greater or less quan- 

 tity, inasmuch as it generally, without interruption, surrounds 

 it on all sides, and passes as well into the mass of rock as 

 into the mass of ore. In cases like those hitherto consi- 

 dered, this substance often affords the clue just alluded to, and 

 attention to it may, under these circumstances, be of direct use, 

 so that we should not in any instance omit its investigation. 



Filons de Contact. — When two different rocks, the one per- 

 haps in the form of a bed, the other in the form of a lying 

 stock, or whatever may be the mode of occurrence, meet un- 

 der the above circumstances, viz., that they do not directly 

 pass into each other, or are not intimately united with each 

 other at the line of junction, but that they are either sepa- 

 rated by mere masses of distinct concretion, or by a foreign 

 substance, the latter, whatever may be its nature, and however 

 unimportant it may at first sight appear, deserves particular 

 attention. It has frequently happened that this substance has 

 not only proved metalliferous, but also that it has consisted chiefly 

 or entirely of ore, and that sometimes it has been of consider- 

 able magnitude. We cannot term such appearances veins 

 or beds,* and cannot compare them with any of the repositories 

 mentioned by authors ; and they would thus seem to have at- 

 tracted less attention than they deserve, although it is known 

 to miners that junctions are important places for their opera- 

 tions. But this does not hold good for all junctions, for when 

 gneiss and mica-slate, or clay-slate, or greywacke, &c, are in 

 contact, the junction is generally accompanied by the appear- 

 ance of transitions in the direction of the thickness ; and hence 

 there is little or nothing to be expected. When, however, 

 granite, especially in amorphous masses, is in contact with one 



* The French term 6uch occurrences da filons de contact. — D'Aubuisson, 

 2ieme e d. t. ii. p. 575. 



