NOT IN PEE-EXISTING CAVITIES. 567 



decreasing- proportionally from the outer limits of the body toward its inte- 

 rior. When these substances are in sufficient!}' large proportion to be visi- 

 ble to the eye, they are seen to be, not in the crystalline condition in which 

 they would be expected to be if they were brought into a pre-existing cavity 

 and then deposited, but in the same granular condition in which they exist 

 in the country rock. Although it may be said that the present outlines of the 

 oxidized ore bodies are not necessarily the same as those of the original sul- 

 phide deposits, it is probable, from the study that has been made of the proc- 

 esses of alteration, that they preserve a general proportion and relation to 

 those outlines, and do not vary from them sufficiently to invalidate the deditc- 

 tion that the original deposits could not have been made in open caves. The 

 deposits in rocks other than limestone consist of metallic minerals and of 

 altered portions of the countr}- rock, in which the structure of the latter can 

 sometimes be still traced, and are not the regular layers of matter foreign to 

 the country rock, which results from the filling- of a pre-existing fissure or 

 cavity by materials brought in from a distance and deposited along- the walls. 



In the case of the still unaltered sulphide deposits of Ten-Mile district, 

 which may reasonably be assumed to have been formed in an analogous 

 way, the arrangement of the particles of the original rock can frequently 

 be seen to be preserved in the metallic minerals, which maintain a certain 

 parallelism with the original bedding planes in the lines defined by minute 

 changes in these minerals. 



Negative evidence is afforded by the absence of that condition of things 

 which would naturally be expected to exist if the ore bodies had been 

 deposited in pre-existing cavities, as has been assumed to be the case by 

 those who have contented themselves with this a priori assumption founded 

 on the theory generally given in text-books, without taking time to study 

 the phenomena as they actually exist. The common character of caves 

 which have been dissolved out of limestone is that their walls are coated 

 with a layer of silt or clay, which has been left undissolved by the perco- 

 lating waters, and that these walls, where undisturbed, have a peculiar sur- 

 face of little cup- shaped irregularities. There is also almost invariably an 

 accumulation at the bottom of the cave of irregular fragments of limestone, 

 which have broken off from its sides or roof. Observation shows us, more- 



