November 22, 1901.] 



SCIENCE. 



787 



explanation in some cases of the relatively 

 even values at different depths? The last 

 question will be first considered. 



In those instances in which the tenor is 

 maintained or practically maintained from 

 the surface to a great depth, the ore is be- 

 lieved to be the result of a single concen- 

 tration by ascending waters. Such ore 

 deposits may continue without any appre- 

 ciable diminution in richness to the lowest 

 limits to which man may expect to pene- 

 trate the earth ; but these are exceptional 

 cases. Even ore deposits which are the 

 result of a single concentration by ascend- 

 ing water may diminish in richness at con- 

 siderable depth. It has been seen that in 

 the fissure at the bottom of the valley on 

 this chart (Fig. 6) the water ascends to 

 the surface. It is evident that the upper 

 part of the fissure receives the greatest 

 supply of water, and this water to a large 

 extent does not penetrate any great depth ; 

 while the lower part of the fissure receives 

 less water, but this water penetrates to a 

 considerable depth. It may happen that 

 the water relatively near the surface 

 traverses the rocks containing the main 

 supply of metals and, therefore, brings the 

 chief contributions of valuable material, or 

 such waters may carry the precipitating 

 agent. In such instances the ore deposits 

 produced by ascending water alone would 

 diminish in richness with depth ; but such 

 decrease would not be likely to be very 

 rapid. Upon the other hand, if the above 

 conditions be reversed, a deposit may in- 

 crease in richness for a considerable depth ; 

 but as a matter of fact this appears to be a 

 very infrequent case. 



As illustrations of the ore deposits of the 

 class produced by ascending waters alone 

 are the copper deposits of Lake Superior. 

 These deposits, while very bunchy and ex- 

 tremely irregular in the distribution of 

 copper, are wonderfully persistent in depth. 

 The copper of the ore was deposited in the 



metallic form. As compared with sul- 

 phides, this material is not readily oxi- 

 dized. In this district the rocks above the 

 level of ground-water are not appreciably 

 weathered. Doubtless there was a belt 

 of weathered material before the glacial 

 epochs, but if so, it has been swept away 

 by ice erosion ; and since the glacial period 

 sufficient time has not elapsed to weather 

 appreciably the rocks which now lie within 

 the theoretical belt of weathering. If there 

 once were in this district an upper belt of 

 weathering in which there were deposits of 

 exceptional richness, this material has been 

 removed. However, in this district, a first 

 concentration by ascending waters was ad- 

 equate, but it is not often that a first con- 

 centration produces deposits of such rich- 

 ness as those adjacent to Calumet and 

 Houghton on Keweenaw Point ; and, in- 

 deed, this is exceptional even in the Keween- 

 awan of the Lake Superior region ; for 

 while concentrations of copper have oc- 

 curred at many points in the rocks of this 

 period, as yet at no other locality have 

 those concentrations been found to be so 

 abundant and rich as to warrant exploita- 

 tion on a large scale. 



I now turn to the question as to the 

 cause of frequent diminution of richness 

 of ore deposits with depth. The explana- 

 tion of the very frequent diminution of 

 value with depth seems to me to lie in the 

 secondary effect of descending currents 

 upon deposits first deposited by ascending 

 waters. Many or most of such ore deposits 

 are believed to be the products of two con- 

 centrations, the first by ascending, the 

 second by descending waters. In this 

 connection it is necessary to call atten- 

 tion to the fact that a large proportion 

 of the deposits which are being exploited 

 are below part of a slope. It may be said 

 that the reason for this is that the low 

 grounds are more difficult to explore and 

 work ; but giving due allowance for this, 



