THE ERUPTIVES. * 359 



the feklspiirs having altered to f)r been replaced by kaolin, chlorite or snui- 

 rag-dite; and the pyroxene having passed over into hornblende, biotite or 

 chlcjrite. 



The alterations have extended farthest in that part of the Iron-bearing 

 member containing the great bodies of ore ; that is, in those parts of the 

 formation which have been subject to the action of percolating waters. 

 This alteration has gone so far that often none of the original minerals 

 remain. All traces of the original stnicture of the rock may even be lost, 

 although frequently the diabasic structure remains imprinted upon the 

 homogeneous almost amorphous material which results from the complete 

 alteration of the rock. 



The strong contrast in the characters of the diabases in the Upper 

 slate and Iron-bearing members shows that environment may be a more 

 important element than age in the preservation of a rock. 



