On the Genesis of Some Scottish Minerals. 209 



cooler waters which are making their way downwards from 

 the surface. This latter cause, in its turn, largely depends 

 upon the form of the surface, as well, also, upon the nature 

 of the rocks — whether these are highly permeable or the 

 reverse. The position of the isogeotherms may be affected 

 also by other causes, even, to some extent, by climate. But 

 the surface configuration at the time when the hot springs 

 are in operation is probably the most important modifying 

 cause now, as it must always have been in the past. I am 

 disposed to attribute to this cause the remarkable disposi- 

 tion of the metalliferous parts of the mineral veins of the 

 north of England, whose upper limits rise to higher levels 

 beneath what are now hills, and sink to a corresponding 

 extent below what are now valleys. This fact is very 

 strongly suggestive of the explanation that those veins have 

 been filled with their latest contents in comparatively-recent 

 geological times, and at a date when the present river courses 

 had been already established. 



There is another point connected with the vertical dis- 

 tribution of ores in certain metalliferous districts which calls 

 for some explanation. It is found, in many cases, that if a 

 series of transverse sections are run across one of these 

 districts where mineral veins are worked, and if the position 

 of the zones where any given ore, such as Galena, occurs in 

 greatest quantity, are carefully plotted on the sections, these 

 points, if connected with each other all round, tend to 

 outline a meniscoid whose convex surface is upward. It 

 is found that if the highest position at which ore occurs in 

 one of these districts be noted, the position of the same ore 

 in the veins around gradually declines in all directions from 

 that point. Of course the vertical extent through which 

 ore may be found in any one vein varies greatly with the 

 nature of the mineral and with the character of the rocks. 

 Galena rarely extends through more than a thousand feet, 

 and usually seems confined to a few hundred. Chalcopyrite 

 has a higher vertical range — perhaps because it was formed 

 at a greater depth. 



The explanation of the meniscoid form of the deposit is 



