METAMORPHOSED GRAYWACKES. 883 



minerals characteristic of that zone are formed. These minerals and their 

 manner of development are, however, identical with those of the gray wacke- 

 schists and graywacke-gneisses next to be considered, and they will there- 

 fore not be here discussed. 



SLATE-GRAYWACKE, SCHIST-GRAYWACKE, AND GNEISS-GRAYIYACKE; OR GRAYWACKE-SLATE, GRAYWACKE- 

 SCHIST, AND GRAYWACKE-GNEISS. 



Where gray wackes are buried deep enough to be in the zone of ana- 

 morphism and are subjected to mass-mechanical action, a schistose or 

 banded rock is produced, to which the terms slate-graywacke, schist- 

 graywacke, and gneiss-graywacke, or graywacke-slate, graywacke-schist, 

 and graywacke-gneiss, are applicable. Where the movement is moderate 

 the slate-graywackes or graywacke-slates are likely to form, but where the 

 mass-mechanical movements are severe schists and gneisses develop. 



The rearrangement of the minerals under mass-mechanical conditions 

 combines the processes of recrystallization and granulation fully described 

 in Chapter VIII (pp. 673-675, 690-696). The minerals which form are of 

 course the heavy anhydrous minerals characteristic of the zone of anamor- 

 phism. The average composition of the minerals in a given case approxi- 

 mates that of the chemical composition of the original ferromagnesian 

 sands, except so far as they have been changed in composition in the belt 

 of anamorphism by dehydration and decarbonation or by injection 



Since one of the essential conditions for the formation of the ferro- 

 magnesian sands is disintegration with but little decomposition, within these 

 rocks the various elements, including the alkalies and alkaline earths and 

 iron, are likely to approximate the average original proportions of the 

 rocks from which the sands were derived. It therefore follows that the 

 ferromagnesian sands are likely to be richer in alkalies, alkaline earths, and 

 iron than the quartz sands or even the quartz-feldspar sands. Since the 

 rocks may contain nearly a full quota of all the elements, almost any of 

 the minerals may form which are characteristic of the deep-seated zone, 

 with the exception of the minerals rich in soda, such as sodalite, nephelite, 

 etc., and the heavy aluminum silicate minerals. Apparently there has 

 always been sufficient depletion in soda to prevent the development of the 

 former. As pointed out on page 900, the latter are likely to develop 

 only where there is an excess of aluminum beyond that required for the 

 formation of the minerals containing the alkalies and alkaline earths. 



