NO. 1546. A PECULIAR FOBM OF METAMOIiPHISM— MERRILL. 549 



though less well-defined section is shown in fig. 3. At first glance 

 such would be pronounced to be a holocr3"stalline rock. It is, in fact, 

 an aggregate of closel}^ interlocking quartz granules with low and very 

 uniform relief, dull colors of polarization, and in the majority of 

 instances a marked rhombohedral cleavage. So striking is this feature 

 that at first the true nature of the mineral was not recognized. 

 Extinctions are often undulatorj^, indicating a condition of molecular 

 strain, and the cleavage lines are themselves at times more or less 

 wavy. The appearance indeed is such as to suggest that the granules 

 have been subjected to pressure while in a putty like or plastic condi- 

 tion. With a high power and between crossed Nicols (fig. 3 of Plate 

 LIl) the rock is seen to be not holocrj-stalline, but to contain compara- 

 tively small colorless interstitial areas, showing by ordinary light a 

 fibrous structure, but which are for the most part completely isotropic 

 between crossed Nicols, and which the chemical analysis suggests may 

 be opal. From this condition the rock passes rarely through more or 

 less vesicular to highly pumiceous forms (Cat. Nos. 76839 and 76840), 

 showing to the unaided e} e all the features of an obsidian pumice, but 

 of a white color. This under the microscope is resolved into a color- 

 less vesicular glass, more or less muddied through dust-like material 

 and showing here and there residual particles of unaltered quartz. 

 The glass does not, however, resemble the glass of a pumice, nor is it 

 like that obtained ])y the artificial fusion of quartz in the geophysical 

 laboratories of the Carnegie Institution. So far as the writer's obser- 

 vations go, it more closely resembles fulgurite glass, formed b}" the 

 lightning striking in siliceous sand. This form, it is well to note, is 

 quite I'are, the material being first met with in what Mr. Barringer 

 has designated as shaft No. 2, and at a depth of 130 feet. A few small 

 pieces were found in digging the open cuts outside of the crater and 

 but one piece lying out on the surface. 



Chemical tests on (I) the unaltered sandstone; (II) what may be 

 called the crystalline variety, the finely laminated stone compared to a 

 decomposed chert, and (III) the pumice, gave Mr. Wirt Tassin results 

 as l)elow: 



(I) TTnaltere<l sandstone [^^^^ ^^- -'* 



lUndet 71 



100. 00 



(11) Altered sandstone . 



fSi.O 98. 6:^ 



AlA 0.18 



FeA 0-10 



Ign 0. 99 



Losa at 100° 0. 30 



100. 20 



