882 A TREATISE ON METAMORPHISM. 



the so-called slate-conglomerates of Logan in the Original Huronian area 

 on the north shore of Lake Huron. a Here there are extensive formations 

 the matrix of which is typical graywacke, and in which are scattered frag- 

 ments, great and small, of the various rocks from which the finer debris 

 was derived. By an increase in the amount and size of the coarse material, 

 the graywackes pass into psephite-conglomerates. Almost equally good 

 illustrations of graywackes are locally found in the great slate formation of 

 the Penokee and Animikie series, although they are here not nearly so 

 extensive as north of Lake Huron. Greikie describes graywacke as a 

 rather common rock in Great Britain. b 



The change from ferromagnesian sands to grits and graywackes is 



believed to take place in an important degree only when the deposits are 



below land areas. The argument in this connection is essentially the same 



as that applicable to the transformation of sands to sandstone and quartzite. 



The openings of the rocks are capillary, and it can 



not be presumed that in the rocks while below the 



water there is any rapid circulation by which material 



may be introduced. Indeed, there is no belt of 



weathering available which can furnish such material. 



But there is one difference between quartzose sand and 



in g serpentmization along fa e ferromagnesian sands. In the latter expansion 



cracks. After Becker. ° x 



reactions may occur while the rocks are below the 

 water. So far as metasomatism there takes place, the processes of hydra- 

 tion, carbonation, and oxidation, all producing increase of volume, would 

 provide material which could be deposited between the grains and thus 

 cement the rocks. To what extent this process has actually taken place 

 below the water must, for the present at least, be a matter of conjecture. 

 But, as first stated, it is believed that the major part of the cementation 

 and metasomatism of the graywackes occurred while the rocks were parts 

 of land areas, for there the conditions are favorable for cementation, viz, 

 vigorous circulation of the ground water and steady contribution of mate- 

 rial to the belt of cementation from the belt of weathering. 



After the graywackes have become cemented by deep burying 

 they may pass into the zone of anamorphism. There metasomatic 

 changes may take place in the rocks, in consequence of which the heavy 



« Logan, W. E., Geology of Canada, 1863: Montreal, 1863, pp. 50-5-1. 



^Geikie, Archibald, Text-book of geology: Macmillan & Co., London, 1893, p. 132. 



Fig. 23.— Graywacke undergo- 



