628 A TREATISE ON METAMORPHISM. 



pyrite and marcasite. The alumina passes into the hydroxides and silicates. 

 As a hydroxide it is a constituent of diaspore and gibbsite. As a silicate it 

 is a constituent of all the zeolites except apophyllite, of all the chlorites, and 

 of the epidotes. 



The carbon of carbonic acid enters as a constituent of the carbonates 

 above mentioned. The silicon of silicic acid enters into quartz, opal, and 

 the silicates above mentioned. The sulphur of hydrosulphuric acid enters as 

 a constituent of the sulphides. 



DISTRIBUTION OF CEMENTING MINERALS. 



It is very noteworthy that the cementing minerals have a strong- 

 tendency to be like the compounds which are cemented. Thus the sands 

 are transformed to sandstones and finally to quartzites by siliceous cement. 

 Where nearly pure limestones and dolomites have been fractured, the dom- 

 inant cementing minerals are calcite and dolomite. If, however, the lime- 

 stone be a very cherty one, the cementing- material may be largely chert. 

 Where the porous rocks are largely silicates, as in the case of the amygda- 

 loids, the cementing materials are mainly silicates, including the zeolites, 

 epidotes, chlorites, etc., but also with these are usually associated important 

 amounts of quartz and carbonates, one being correlative with the other ; for 

 carbonation of the silicates, forming carbonates, liberates silicic acid, which 

 may separate as quartz. 



These phenomena are beautifully illustrated by the amygdaloids of 

 the Keweenawan series described by Pumpelly" and Irving, 6 and by the 

 Crystal Falls volcanics described by Clements." 



In many cases not only are the silicates cemented by silicates, but the 

 cementing- minerals are like the dominant minerals cemented. For instance, 

 the feldspathic sandstones may be cemented by feldspar, as in the case of 

 the Keweenawan sandstones of Eagle Harbor. 1 ' Where hornblende is 

 abundant the cementing material may be largely hornblende, as in the case 

 of the Cacaquabic tuffs of northeastern Minnesota. 



" Pumpelly, Raphael, Metasornatic development, of the copper-bearing rocks of Lake Superior: 

 Am. Acad. Arts and Sci., vol. 13, 1878, pp. 253-309. 



''Irving, R. D., The copper-bearing rocks of Lake Superior: Mon. U. S. Geol. Survey, vol. 5, 1883, 

 pp. 87-91, 134-139. 



c Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan: 

 Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 73-154. 



''VanHise, cit., Bull. 8. 



