212 GRANITES AND GRANITIC VEIN-STONES. [XL 



plete, while the space within either remains empty, or is filled 

 with other minerals, often unsymmetrically arranged. This 

 condition of things is rendered intelligible by the forma- 

 tion of similar hollow crystals during the cooling of certain 

 saline solutions, as for example potash-nitrate. Small hollow 

 prisms of red and green tourmaline, closely resembling the 

 hollow nitre crystals, are common in the well-known gra- 

 nitic vein-stone of Paris, Maine. I have elsewhere referred 

 to the formation of such moulds or skeleton-crystals as having 

 taken place in vein-cavities, and as serving to explain many 

 cases of enclosure of mineral species. (Address to the A. A. A. 

 S., Indianapolis, 1871. Paper XIII. of the present volume.) 

 In addition to the examples there cited, the Laurentian vein- 

 stones afford some curious cases. Thus a prism of yellow 

 idocrase half an inch in diameter from a vein in GrenviUe, 

 Ontario, composed chiefly of orthoclase and pyroxene, is seen 

 when broken across to consist of a thin shell of idocrase filled 

 with a confused crystalline aggregate of orthoclase, which 

 encloses a small crystal of zircon. In like manner large crys- 

 tals of zircon from similar veins in St. Lawrence County, New 

 York, are sometimes shells filled with calcite. 



42. The rounded forms of certain crystals in the Lauren- 

 tian vein-stones were, I believe, first noticed by Emmons, who 

 observed that crystals of quartz imbedded in carbonate of lime 

 from Kossie, New York, have their angles so much rounded 

 that the crystalline form is nearly or quite effaced, the surfaces 

 being at the same time smooth and shining. This appearance 

 is occasionally observed in other localities, and is not confined 

 to quartz alone, crystals of calcite and of apatite sometimes 

 exhibiting the same peculiarity. At the same time the ortho- 

 clase, scapolite, pyroxene, and zircon, which are associated with 

 these rounded crystals, preserve all their sharpness of outline, 

 as was observed by Emmons for the orthoclase in contact 

 with the crystals of quartz just described. He suggested that 

 the rounded outlines of these crystals were due to a partial 

 fusion, although he did not overlook a fact which ronlrs 

 this explanation untenable, namely, that the species presenting 



