724 A TREATISE ON METAMORPHISM. 



stages of gradation between true igneous injection and aqueous cementation, 

 and all the various phases of pegmatization may thus be fully explained. 



This idea of continuity was first suggested to me by the phenomena 

 observed in the schistose rocks surrounding the intrusive core of the Black 

 Hills. Remote from the intrusives the sedimentary rocks are slates; adja- 

 cent to it they are schists and gneisses. The core rock is a great batholith 

 of granite, 11 km. broad and 18 km. long. Besides this central mass there 

 are, to the southwest, a number of smaller masses, from 2 to 6 km. 

 long, which may be connected below with the greater mass. From the 

 central mass great quartz-feldspar dikes radiate. In passing- away from the 

 core the dikes become smaller and have a less typical form; at the same 

 time the material assumes the appearance which we ordinarily denominate 

 pegmatitic. These veins might be considered true igneous injections. Still 

 farther away the pegmatitic masses begin to have vein-like characters — 

 that is, there is a rough concentration of the material in different layers par- 

 allel to the walls. Still farther away a true banded-vein structure is found. 

 Yet farther away feldspar becomes less and less important in the veins and 

 the quartz more abundant, until remote from the granite the impregnating 

 material is mainly quartz. Within the intruded rock, adjacent to the 

 granite, there is also an extensive development of feldspar, giving the rock 

 a banded appearance and changing it from a schist to a gneiss. If one 

 examined only the outer zone, in which the quartz cementation series 

 occur, one would not doubt that they are ordinary deposits from ground 

 water; if one examined the dike-like masses of the inner zone adjacent to 

 the great granite mass, one would not doubt that they are true igneous 

 injections; but in passing back and forth between the two, one observes that 

 there is every gradation between them, and is driven to the conclusion 

 that true igneous injection was predominant adjacent to the granite, that in 

 the central zone aqueous and igneous agencies were about equally impor- 

 tant, and that in the outer zone aqueous agencies were predominant. It is 

 impossible to believe that the larger dikes — plainly offshoots of the central 

 batholith — are not igneous injections; it is equally impossible to believe 

 that quartz veins remote from the granite are dikes, or that the ordinary 

 granite magma has penetrated to considerable distances between the indi- 

 vidual grains of the schist and thus impregnated it with feldspar. 



From the foregoing it is clear that under the general term pegmatite 



