Invasion to Regional Metamorphism. 263 



out this line of research; but on the contrary merely to 

 show that the metamorphic phenomena associated with 

 the roofs of orogenic batholiths, in order to be understood, 

 must be analyzed into the two phases of the rise and 

 decadence of magmas. 



Alterxatiox of Injection and Mashing. 



This phenomenon may be made more impressive by the 

 description of certain specific formations rather than 

 by dealing wholly in general statements. The writer has 

 studied these relations especially in the Laurentian 

 Becket gneiss of southwestern Massachusetts and in 

 various Paleozoic batholiths and intrusions in Connec- 

 ticut. 



The Becket gneiss has been described by Emerson,^^ 

 but the following statements are planned to bring out in 

 brief form those features significant under the present 

 topic. The dominant phase is a biotite granite gneiss, 

 almost massive in some localities, in others strongly 

 banded. The gneissoid phases of this rock show consid- 

 erable granulation and only a moderate amount of 

 recrystallization. On a fresh fracture the rock shows 

 a grain like granulated sugar, and the biotite is scattered 

 with a pepper and salt effect, not markedly segregated 

 into planes. On cliff faces the biotite weathers out, the 

 surface becomes white and siliceous and gives rise to 

 the name of white gneiss. The banding is probably 

 mostly due to non-uniform compression and flowage 

 during crystallization, though considerable granulation 

 after complete solidification seems to have occurred. 



From this rather uniform facies of the gneiss there are 

 passages into areas of highly variable banding, the 

 banding being of all scales of magnitude from a few inches 

 to hundreds of feet. The bands consist of amphibolites, 

 hornblende, diorite gneisses, biotite granite gneisses, 

 acidic granites, and pegmatites. The hornblende layers 

 in many instances are intimately associated with bands 

 of pegmatite and aplite, feathering into the acidic 

 material and' swelling into lenses in a way to show that 

 the two were simultaneous differentiates rising through 

 the foliated rock and crystallizing separately from an 

 aqueous solution. The amphibolite, often developed as 

 a ribbon gneiss, is most abundant in contact zones against 



'° B. K. Emerson, U. S. Geol. Survey, Bull. 159, 1899. 



Am. Jour. Sci.— Fifth Series, Vol. I, No. 8.— March, 1921. 



18 



