METAMORPHIC STUDIES 607 



primary igneous rocks, from which all metamorphism presumably 

 starts, and is essentially a by-product of the metamorphic cycle. 

 The difference between schists, gneisses, and slates, on the one 

 hand, and igneous rocks on the other, represents a gap which has 

 not been closed by the metamorphic cycle. The schists, gneisses, 

 and slates represent types which are clearly nearer the composition 

 of the original igneous rock than many of the rocks, particularly 

 the sediments, which have been anamorphosed and recrystallized 

 and have undergone rock flowage. 



This convergence toward igneous rock composition often reaches 

 a point where chemical criteria are not sufficient to determine 

 whether or not the primary rock was igneous or sedimentary, the 

 resulting composition being so nearly that of an igneous rock. If, 

 along igneous contacts, a considerable amount of material has been 

 completely fused, the anamorphic product may take on the char- 

 acter of an igneous rock and in such places the cycle is closed. 

 But just as sandstones or limestones may be regarded as products of 

 the cycle, some of which never go back to their primary condition 

 of igneous rocks, so the schists and gneisses are by-products, which 

 do not go back to the condition of igneous rocks under ordinary 

 conditions of anamorphism. Whether the cycle in its larger aspects 

 is completely closed, so far as the great mass of products is con- 

 cerned, is a very doubtful question which has been discussed in 

 a previous paper.^ 



' Jour. GeoL, XX (191 2), 353-61, 



