394: Armstrong — Zircon as Criterion of 



been absolutely stable under the conditions of tempera- 

 ture and pressure to which it was subjected. However, 

 the evidence collected during this investigation would 

 speak against this assumption. The zircons of igneous 

 as well as sedimentary gneisses were found to show a 

 peculiar rough surface, blisterlike in appearance, the 

 nature of which it was impossible to determine under the 

 microscope. In the belief that this blisterlike appearance 

 is due to a coating of silica, several grains were treated 

 with warm hydrofluoric acid, but without result. The 

 biisterlike covering has either destroyed, or, in any case, 

 now completely obscures the original crystal face ; how- 

 ever, rotation of the grain under the microscope brings in 

 view a great number of apparently irregularly placed, 

 small, and more or less round faces of rather dull luster 

 which appear to have developed on and within the 

 blistered surface. Possibly the latter represents a 

 decomposition product of zircon and may be one of the 

 hydrous forms of this mineral, e.g. malaconite, described 

 by Doelter. 7 A zircon, isolated from the igneous Maro- 

 mas gneiss of Connecticut consisted of a splinterlike 

 plate, extremely irregular in outline but having all its 

 edges rounded by the blisterlike covering just mentioned. 

 Similarly, parts of prisms were found in other igneous as 

 well as sedimentary gneisses, one of their ends showing 

 a pyramidal termination, the other a conchoidal fracture, 

 the latter modified in exactly the manner as described 

 under the Maromas gneiss. It should be of greatest 

 interest to determine whether the composition of this 

 blisterlike surface is either identical with or related to 

 that of the zone of corrosion of Doelter 's, previously 

 mentioned. Our conception of the stability of zircon may 

 then, perhaps, be greatly modified. 



That zircons are deformed under metamorphic stress 

 can be confidently asserted. From an igneous gneiss the 

 writer separated a grain, having a curved sausage shape 

 and showing on its concave side a single crystal face, 

 embedded, so to speak, in the blistery growth which 

 covered the rest of the grain (fig. 4). Others show pear 

 or club shapes, and many are such perfect spheres that 

 they would apparently fully justify the belief that they 



7 Op. eit. 3 p. 136. 



