48 A. A. Young— Sands of the Potsdam Sandstone. 



examined the lack of homogeneity becomes apparent at first 

 glance. The crystals — the most complete as well as those least 

 so — are found to inwrap fragments of varying size, contour and 

 structure. When viewed by reflected light, on a dark back- 

 ground, the crystallized exterior is barely discernible as a thin 

 transparent envelope, while the enclosed fragment appears, 

 often semi-opaque and has a sharply defined outline. 



Generally the longer and shorter diameters of the whole 

 grain accords with those of the enclosed fragment; but in 

 some they are reversed so that the narrowest part of the frag- 

 ment lies in the line of the greatest length of the grain of sand. 

 Viewed alternately by reflected and transmitted light, one sees 

 apparently two different objects. 



The greatest thickness of the crystal envelope which I have 

 yet measured is '004 in. Ordinarily the maximum thick- 

 ness—at the extremities of the pyramids— is from -001 in. to 

 •0015 inch; but over much of the surface the envelope is so 

 thin as to be but barely appreciable with a power of 150 diame- 

 ters. In this envelope bubbles or cloudiness are very rare. 



The enclosed grains vary much in contour. Many are 

 rounded in portions of the outline, though few are symmetri- 

 cally so. Some are as sharp-angled and jagged as fragments- 

 of freshly broken rock. Sometimes their surfaces show by 

 light groovings of a length double or quadruple their 

 breadth, either in single lines or sets of lines, or in lines cross^- 

 ing at varying angles, and in the nuclei of crystals of only '005 

 in. in length, as well as in larger. They are furrows Father 

 than scratches, as though the fragment had been rolled with a 

 dragging motion over cutting points. This structure appears- 

 also in the nuclei of the crystals of the St. Peter's sandstone. 

 Such facts throw light on the previous history of the in 

 fragments. These groovings are also finely exhibited in sand r 

 from friable rock of a higher horizon of Potsdam, in which 

 only occasional traces of a crystal envelope appear. 



In structure the enclosed fragments show great diversity. 

 Homogeneity is the exception. In some a general cloudiness 

 exists, or in I babbles; and in others there are 



planes or belts of bubbles. Aside from these are other note- 

 worthy enclosures. Some grains are traversed by needle-like 

 lines, suggestive of rutile threads in quartz, which occasionally 

 terminate abruptly against the inner surface of the tra 



e envelope. In some grains these threads form a 

 parallel system, and in others they are set at all angles with one 

 another. Very rarely they are curved or bent. In one grain, 

 only -02 in. x' '015 in., nearly fifty such threads occur. I have 

 found this ru tile-like structure also in the samples of the Pots- 

 dam sand from various horizons; in sand from the Pictured, 

 rocks of McGregor, Iowa, and in the St. Peter's sand. 



