F. D. Adams — Melilite-oearing Rock. 



275 



"When both serpentine and magnesite are present as altera- 

 tion products in the same olivine individual their boundaries 

 against one another are sharply defined and the two minerals 

 do not occur mixed together. The magnesite seems to have 

 been formed before the serpentine, as it frequently occupies 

 the outer part of the crystal while the inner portion is altered 

 to serpentine. In these cases the alteration to magnesite 

 appears to have gone on to a certain point and then ceased, 

 while the rest of the olivine was subsequently altered to ser- 

 pentine in the usual manner. Two of these altered olivine crys- 

 tals are represented in the acconqDanying figure (fig. 1). In one 

 of them magnesite alone is present 

 as a decomposition product, while 

 the other is altered to magnesite and 

 serpentine. 



It is believed that this peculiar 

 alteration of olivine has not been 

 hitherto observed. A very similar 

 if not identical alteration product, 

 however, is seen in the olivine occur- 

 ring in certain fine grained nephe- 

 line-bearing dykes from the Mon- 

 treal mountain, and in Sagvandite* FlG L M ^ t] 

 we probably have a somewhat similar 

 alteration of bronzite to magnesite. 



Pyroxene. — The pyroxene which 

 is one of the most abundant con- 

 stituents in the rock is also in many respects remarkable. Two 

 varieties are present, one forming the large phenocrysts and the 

 other occurring very abundantly in the groundmass. The 

 phenocrysts which, as mentioned above, are often very large, 

 are usually well crystallized but sometimes have more or less 

 rounded forms. The mineral is fresh and unaltered even when 

 the other constituents have undergone considerable decomposi- 

 tion even where cavities filled with secondary minerals have 

 eaten their way into its substance the pyroxene around their 

 border is quite fresh. When crystallized, basal sections pre- 

 sent eight sided forms showing a development of both pris- 

 matic and pinacoidal forms, with good cleavage parallel to the 

 former and extinction parallel to the latter. It is monoclinic 

 and the maximum extinction observed in the zone of the ortho- 

 pinacoid and clinopinacoid was 42.° It is colorless and never 

 shows more than mere traces of pleochroism, it has however a 

 strong dispersion of the bisectrices so that when a thin section 

 is revolved between crossed nicols until the point of maxi- 



*H. Kosenbusch: " Ueber den Sagvandite," Neues Jahrbuch fur Mineralogie, 

 etc.. 1884, p. 195. 



magnesite (stipple) and to mag- 

 nesite and serpentine (vertical 

 lines). The white spaces are 

 unaltered olivine. 



