904 A TREATISE ON METAMOEPHISM. 



gneisses of the pelite order results, thus affording the best examples of 

 rock cleavage. 



Where the shales are rather homogeneous in texture, slates and schists 

 form. Where, however, the original muds had alternations of coarse and 

 fine material, or alternations of bands of material of different chemical 

 compositions, these differences are apt to be maintained, to some extent at 

 least, in the metamorphosed rocks, and thus a banded gneissic structure 

 be produced. Each band of such a gneiss corresponds approximately in 

 chemical composition to that of the original rock, and is mainly composed 

 of the particular minerals, in the approximate proportions, which may be 

 produced from the band of mud. 



Since the muds are the most extensive of mechanical sediments, it 

 naturally follows that there are great formations composed of their meta- 

 morphosed equivalents — slate-pelites, schist-pelites, and gneiss-pelites. As 

 illustrations of coarse pelite-schists having a very widespread occurrence 

 may be mentioned the Hudson schists of New England and New York, of 

 Silurian age." Other equally good illustrations of very extensive schist- 

 pelite formations are those of the Upper Huronian formation of the Lake 

 Superior region. 6 Many other illustrations might be mentioned, but it is 

 hardly necessary to take the space to do this. 



IGNEOUS KOCKS. 



As yet we have no classification of the igneous rocks to which there is 

 general agreement. In this respect the igneous rocks stand on a basis 

 different from that of the sediments. As no classification of the igneous 

 rocks is agreed upon, since petrographers themselves have not decided as to 

 which rocks shall have specific names, and as it is agreed that there are 



"Merrill, F. J. H., Metainorphic crystalline rocks in New York City district: Geologic Atlas U. S., 

 folio 83, TJ. S. Geol. Survey, 1902, p. 4. 



''Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin: 

 Mon. U. S. Geol. Survey, vol. 19, 1S92. pp. 296-345. Van Hise, C. R., and Bayley, W. S., with 

 H. L. Smyth, The Marquette iron-bearing district of Michigan: Mon. TJ. S. Geol. Survey, vol. 28, 

 1897, pp. 444^159. Clements, J. Morgan, and Smyth, H. L., with W. S. Bayley and C. R. Van Hise, 

 The Crystal Falls iron-bearing district of Michigan: Mon. TJ. S. Geol. Survey, vol. 36, 1899, pp. 164-174. 

 Bayley, W. S., The Menominee iron-bearing district of Michigan: Mon. TJ. S. Geol. Survey, vol. 46, 

 1903, pp.. 320-321, 462-488. Leith, C. K., The Mesabi iron-bearing district of Minnesota: Mon. TJ. S. 

 Geol. Survey, vol. 43, 1903, pp. 168-177. Clements, J. Morgan, The Vermilion iron-bearing district of 

 Minnesota: Mon. TJ. S. Geol. Survey, vol. 45, 1903, pp. 391-396. 



