768 C. R. VAN HISE 



which originally had, as result of the early concentration, a much 

 wider vertical distribution. 



To the foregoing classification objections will at once be made : 

 It will be said that there are no sharp dividing lines between the 

 groups and classes. To this objection there is instant agreement. 

 Transitions are everywhere the law of nature. It is v/ell known 

 that there are gradations between different classes of rocks, 1 and 

 this statement applies equally well to ore deposits. I even hold 

 that there is gradation between ore deposits which may be 

 explained wholly by igneous agencies and those which may be 

 explained wholly by the work of underground water or by 

 processes of sedimentation. 



I have elsewhere held that there is complete gradation between 

 waters containing rock in solution and rock containing water in 

 solution. 2 If there be no sharp separation between water solu- 

 tions and magma it is probable that this is also true in reference 

 to ore deposits of direct igneous origin and those produced by 

 underground water. There may be ore deposits in which water 

 action and magmatic differentiation have been so closely associ- 

 ated that one cannot say whether the resultant ore deposit is 

 mainly a water deposit or mainly a magmatic deposit. But for 

 the vast majority of ore deposits, if I properly apprehend the 

 relations, the broad general statements which I have made apply. 

 Ordinarily there is little difficulty in discriminating between 

 veins and dikes, the first representing crystallizations from water 

 solutions, the second crystallizations from magma. There are 

 few cases where the discrimination in reference to ore deposits 

 is not easy. While gradations between water deposited ores and 

 igneous ores are uncommon, gradations between the different 

 classes of ore deposits formed by underground water are common. 



Ores which have received a first concentration by igneous 



agencies or by processes of sedimation are sure to be reacted 



upon by the circulating underground waters, and thus a second 



x The Naming of Rocks, by C. R. Van Hise : Journ. of Geol., Vol. VII, 1899, 

 pp. 687, 688. 



2 Principles of North American pre-Cambrian Geology, by C. R. Van Hise : 

 Sixteenth Ann. Rept. U. S. Geol. Surv., 1894-5, Pt. I, p. 687. 



