414 R. A. DALY META^MOEPIIIS^r AND ITS PHASES 



voluminous granitic niagnia of post-Cambrian age has been at once canse 

 and effect of gas-tliermal alteration. On the other hand, conditions 

 special to the earlier Precambrian may, as Sederholm suggests, have 

 caused palingenesis in the older terranes, tlirough the cooperation of heat 

 and rising gases. In fact, tliere is some ground for the hypothesis that 

 the earth's original crust was changed by load-contact metamorphism, 

 palingenesis, and lit-par-lit injection during its original, slow formation. 

 The geology of the Precambrian complexes seema to indicate for the 

 original crust: (a) an average chemical composition like that of common 

 granite; (h) a general gneissic structure, due to load metamorphism in 

 tlie presence of abundant water and a steep thermal gradient; and (c) 

 injection of countless granitic sills along the new planes of foliation, fol- 

 lowed l)y more or less perfect load metamorphism of the sills themselves. 

 A very thin surface shell of massive granitic or rhyolitic rock may have 

 covered the thickening crust, but, below the depth of a few hundred 

 meters, that crust would be a composite, as described. Perhaps Seder- 

 liolm (1910, page 134) is right in assuming the possibility of actual 

 representation of the original crust in the older Precambrian formations 

 of Fennoscandia and Canada. 



Certain pegmatites and perhaps certain veins of aplitic constitution 

 may have been formed by the "selective solution'' (Lane, 1913, page 

 704) of some components of a rock-mass which has undergone ordinary, 

 tliough intense, metamorphism, dynamic or static* Small bodies of such 

 new magma, forced out of the parent formation and injected into other 

 rocks, may cause a little contact metamorphism, but the magmatic bodies 

 themselves are by-products of regional metamorphism and belong to the 

 held of ultra-metamorphism. 



iiccording to the proposed definition, exomorpliic changes, leading to 

 new crystallizations near igneous contacts, must, as usual, be treated 

 under metamorphism, while endomorplnc solution of country rock is 

 another example of ultra-metamorphism. Thus hybrid rocks properly 

 fall in the igneous rather than the metamorphic class. 



SUMMAEY 



Tlie problem of rock altej'ation below the earth's shell of weathering is 

 immeasurably complex. The kinds of change are many. The necessity 

 of their indefinitely detailed discussion is one of the most insistent duties 

 of a field geologist. Fruitful discussion depends on names and defini- 

 tions. The most used and most important name is "metamorphism," the 



* Since this paper went to press, Ilolmquist's 191G ai-ticle on the Swedish Archean has 

 come to America. Holmquist (page 141) there clearly states his helief in the ultra- 

 inetamorphic origin of many Archean pegmatites and aplites. 



