METAMORPHOSED GRAYWACKES. 885 



The newly developed minerals show a marked tendency to similar 

 dimensional arrangement. This tendency is very strongly marked with the 

 micas, is important with the hornblendes, and applies to a considerable extent 

 to quartz and the feldspars. Corresponding with the dimensional arrange- 

 ment of the mica there is an eminent cleavage. There may be a cleavage 

 corresponding to the dimensional arrangement of the hornblendes and the 

 feldspars. This dimensional arrangement and mineral cleavage combined 

 give the rocks a slaty or schistose structure, depending upon the coarseness 

 of the mineral particles. (See pp. 778-779.) 



Where the original ferromagnesian sands were banded in consequence 

 of alternation of coarse and fine material, or of alternation of different 

 combinations of the mineral constituents, this banding may be preserved 

 and thus a gneissic structure be produced in the recrystallized rocks. 

 Where there was a band in which quartz was very abundant, the lamina? 

 are composed largely of quartz; where there was a quartz-feldspar band 

 there is likely to be a lamina composed of quartz, feldspar, and mica; 

 where there was a band in which were abundant ferromagnesian minerals 

 there are likely to be lamina? in which hornblende, feldspar, * mica, and 

 quartz are all plentiful. Of course, the bands are not sharply separated 

 from one another, and there is likely to be subordinate amounts of all the 

 minerals which occur in the rock in each of the bands. The preservation 

 of the banded structure is due to the fact, fully explained in Chapter VIII 

 (pp. 764-766), that during the alterations which take place in the zone of 

 anamorphism migration of large quantities of material does not take place 

 for more than exceedingly short distances. 



It will be seen later that the gneisses of igneous origin usually contain 

 hornblende. The sedimentary gneisses belonging to the family under 

 consideration usually contain hornblende, and therefore are more likely to 

 be confused with the gneisses of igneous origin than with the gneisses of 

 the quartz-sand or quartz-feldspar-sand families. (PL X, C.) However, 

 in many cases the average chemical composition of the rocks and the 

 arrangement and structures of minerals enable one to discriminate g-neisses 

 of the family here considered from those derived from the igneous rocks. 

 But, apparently, where the sedimentary rocks are adjacent to great 

 intrusive igneous masses of hornblende-granite the sedimentary rocks may 



