228 Redeic8 — Dr. J. Petersen on Cheviot Rocks. 





I. (P.) 



11. (T.) 



SiOs ... 



... 52-53% ... 



... 63-06 



A1203 ... 



3-38 



4-90 



FeO ... 



... 9-89 



... 16-62 



MgO ... 



... 26 66 



19-64 



CaO 



... 6-19 



4-09 



HoO ... 



... 0-26 



— 



98-91 98-31 



The author concludes that his analysis indicates bronzite. Com- 

 paring the two analyses, he considers that the differences may be 

 accounted for on the assumption that I analyzed a mixture of the two 

 pyroxenes. This view appears to me improbable for two reasons : 

 (1) the microscopic examination of the material which I analyzed 

 showed it to be remarkably pure, although, as the author states, I 

 took no special steps to remove the monoclinic augite ; (2) the amount 

 of lime in my analysis is less than that in his, a fact which seems in 

 itself conclusive against the view which he suggests. A comparison 

 of the two analyses to my mind leads rather to the conclusion that 

 the mineral varies in composition in different portions of the same 

 rock, or at any rate in different specimens from the same district. 



His analysis is especially interesting to me, because it tends to 

 confirm an opinion that I was led to form after isolating the pyroxene 

 of the Steinerne Mann rock, viz. " that lime enters more lai'gely into 

 the composition of some of these rock-forming hypersthenes than 

 the earlier analyses of this mineral would lead one to expect." 

 (Geol. Mag. 1883, p. 3d6.) 



The monoclinic augite is then described. The boundaries in the 

 prismatic zone are not so sharp as in the bronzite ; indeed, the mineral 

 occurs rather in the form of crystalline grains. Longitudinal sections 

 show parallel cleavage cracks ; cross sections indicate both prismatic 

 and pinacoidal cleavage. There is no trace of pleochroism. The 

 maximum extinction angle in the prismatic zone is 44° (this is 

 exactly the figure obtained by myself). 



The polarization tints as a rule belong to a higher position in 

 Newton's scale than those of the bronzite. The rod-like interposi- 

 tions, so frequent in the bronzite, are entirely absent from the mono- 

 clinic augite. Twinning parallel to the orthopinacoid is fi-equently 

 present. 



The author then makes one very interesting observation which I 

 can fully confirm, although I had not observed it at the time my 

 paper was written. Intergrowths of augite and bronzite are not 

 unfrequent.^ The bounding surfaces are irregular, and the cleavage 

 cracks are parallel to each other in the two pyroxenes. 



The bronzite was formed before the augite according to the author. 

 In one instance, he observed a bronzite crystal completely surrounded 

 by augite. 



The felspar belongs to two, if not three generations. The larger 



' Tripke recognized intergrowths of enstatite and diallage in the olivine inclnsions 

 in basalt. Orthopinacoidal lamellaj of diallage are in this case arranged parallel to 

 the macropinacoid of the enstatite, Naumaun-Zirkel, Eleinente der Mineralogie, 

 1881, p. 598. 



