192 Derby — Accessory Elements of Itacolumite, etc. 



surfaces, etching has in several cases been noted. In the cases 

 of quartz and of staurolite in which it is most pronounced, the 

 etching seems to have taken place during the decay of the 

 rocks. In the few cases in which it has been rather indis- 

 tinctly observed on zircons, it. appears to be attributable to 

 magnetic absorption, to which also the rounding of the angles, 

 when found in eruptive rocks, is probably due. Whether in 

 more completely metamorphosed rocks these changes may ren- 

 der zircon also useless as an evidence of clastic origin, is a 

 question to be investigated for which material is unfortunately 

 not at hand. The observations thus far made on siliceous and 

 argillaceous rocks, both eruptive and clastic, give a hope 

 that it may prove to be a reliable witness in the great majority 

 of cases. The occurrence of macroscopic zircons in calcareous 

 rocks, however, indicates either an introduction of new mate- 

 rial or a migration and concentration of the material of micro- 

 scopic clastic grains that occur in most limestones when not 

 absolutely pure. An investigation of this point in some of 

 the well-known zircon localities would be of interest. 



