THE EEUPTIVES. 



357 



extremely altered phases; that the great majority of the soapstones retain 

 a distinct dialjasic structui'e ; and that the alterations of the minerals from 

 their fresh condition to complete decomposition are traceable in all its 

 stages. With the decay of the rock there has come about, as would be 

 expected, a very considerable change in composition. 



The analyses below are by Mr. Thomas M. Chatard of the chemical 

 laboratory of the United States Geological Survey. No. 1 (specimen 12880) 

 is a fresh diabase, from Sec. 13, T. 47 N., R. 46 W., Michigan. The felds- 

 par of this diabase is labradorite. No. 1 of page 352 being from this rock. 

 No. 2 (specimen 12878) is from the same dike, where the alteration has ex- 

 tended to a middle stage; and No. 3 (specimen 12966) is typical soap rock 

 from the Aurora mine, in the NE. J of the SW. ^ Sec. 23, T. 47 N., R. 47 



W., Micliigan. 



Analysen of diabases and soapstone. 



H,Oatl05o 



HjO at red heat. 



COj 



S03* 



P,Ob 



SiOj 



TiOj 



AljOj 



Fe,0, 



CrjO, 



FeO 



NiO(CoO) 



MnO 



BaO 



CaO 



MgO 



KjO 



Na,0 



' SOj calcul.it 0(1 from BaO found, a^ tlii-i latter probably exists as BaSo^. 

 t Al^Oj is prctbably a little liigh nwiny; to alkali relaiued by titanic .-icid. 



The analyses indicate that in the decomposition of these rocks the 

 minerals become hydrated; that the silica lessens in quantity; that the rela- 

 tive proportion of alumina is largely increased ; that the calcium, magne- 

 sium, and iron i)rotoxide are almost wholly removed ; and that the relative 

 proportion oi' titanic oxide is increased, this probably being due to the fact 



