'2Q6 PROCEEDINGS OF THE AMERICAN ACADEMY 



treatment during twenty-four hours with cold muriatic acid, which 

 removed considerable iron from the altered chrysolite. 



On etching thin sections with acetic acid, during a period of forty- 

 eight hours, it was found that not only was calcite removed from the 

 impellucid veinlets, but also extensively from the representatives of the 

 pyroxene. Tiie action of the acid revealed the presence of numerous 

 cracks in the latter, which were filled with a colorless, more or less 

 transparent, and double-refracting substance not acted upon by the 

 acid. On examining the etched section by reflected light, and after- 

 wards in polarized light, the former positions of the pyroxene individ- 

 uals were found to be occupied by a thoroughly honeycombed mass ; 

 .the crystalline substance that had been formed in the cracks now form- 

 ing the partitions between cells, which contained an apparently con- 

 fused granular mass of white translucent substance. On the thinner 

 edges of the section the cells were often entirely empty, perhaps owing 

 to mechanical action in washing after etching. 



The plagioclase remained, as before the treatment with acid, appar- 

 ently fresh, except in immediate contact with the calcite veinlets, and 

 also where former interpositions of glass base had been changed to a 

 greenish double-refracting substance. 



That the ferric oxide which stains the pseudomorphs after chrysolite, 

 and also occupies capillary cracks in other constituents, was formed 

 from the iron in the chrysolite, seems to be apparent from the fact that 

 it exists in the less altered form of the rock where the pyroxene is 

 perfectly fresh. 



That this staining is older than the calcite appears from the fact that 

 the calcite veinlets cut the stained cracks, and are never themselves 

 discolored. But the magnetite appears to have been oxidized at a 

 later date without spreading a discolorization. 



The chrysolite was the first to suffer, for we find it wholly changed 

 in the less altered form of the rock. The change, possil)ly, cousitited 

 in a hydration, accompanied by separation of iron. 



There occur seams jJg inch thick of a green, non-dichroitic chlorite, 

 which shows lamellar aggregate polarization. These seams are trav- 

 ersed by the calcite veinlets, and are, therefore, older. 



The quartz, both in the larger vein fragment and in the veinlets, 

 seems to be younger than the calcite, though the evidence is not as 

 positive as could be wished. In a section cut from the vein fragment, 

 I observed a quartz crystal cut nearly perpendicular to the principal 

 axis, and surrounded by calcite : it seems that the growth of the quartz 

 was interfered with ; for its development is marked by successive con- 



