640 



ERUPTIVE BOCKS FROM MONTANA— MERRILL. 



comparison, chrome diopsides out of tlie peridotites of (ii) Lake of 

 Llierz, (III) Dillgegeud, and (iv) Piedmout, as given by Teall.* 



SiO, . 

 Al.,03 

 GrjOs 

 re.,03 

 FeO . 

 MnO. 

 CaO . 

 MgO. 

 TS..JO . 

 Na.,0 

 HoO . 



A bulk analysis of the rock yielded Dr. Chatard as follows: 



SiOj. 

 TiOj. 



Al,03 

 Cr^Os 

 Fe.,03 

 FeO.. 

 MnO. 

 CaO . 



Per cent. 



46.90 

 0.41 

 0.44 



10.17 

 0.33 

 1.22 

 5.17 

 0.10 

 6.20 



MgO 



KvO 



ISTa-O 



H.Oat 120= C. 

 HjO at red heat. 



Specific gravity 



Per cent. 



20.98 

 2.04 

 1.16 

 1.04 

 4.38 



100. 54 

 2.86 



The composition, as above indicated, is quite unlike that of any rock 

 I have yet seen described. So far as indicated by the silica percentage, 

 the rock might belong to the diabase or basalt grouj), but the magnesia 

 percentage is far higher than I have ever seen reported in rocks of 

 this class, and from either of which it differs structurally. It is equally 

 dillicult to consider the rock a peridotite, since not only is the silica 

 percentage above that of a normal peridotite, but the high percentage 

 of potash and soda (confined wholly to the minerals of the groundmass) 

 indicate a very considerable quantity of sanidin and soda-lime feldspar. 



To gain a further insight into the composition of the rock, fragments 

 from the same mass as that used in the above bulk analysis were subse- 

 quently sent to the laboratory with the request that an analysis be 

 made of that portion soluble in hydrochloric acid. The results are given 

 below. 



Amount of rock soluble in HCL 48.7191 iier cent. This yielded: 



SiOj .... 

 Al.Oj... 

 Fe^O., ... 

 FeO .... 

 MnO.... 

 CaO . . . . 

 MgO.... 

 Alkalies 



Per cent. 



42.87 



7.98 



4.53 



8.78 



Trace. 



2.07 



33.69 



Trace. 



99.92 



Ratio. 



714 



.122] 



037^ l.OOl 

 842 



*Briti8h Petrography, p. 89. 



