830 



SCIENCE. 



[N. S. Vol. III. No. 75. 



ON TEE DETECTION OF GLACIAL STBIJE IN 

 REFLECTED LIGBI. 



It is known that in many regions of gla- 

 ciation, owing to the softness or attitude of 

 the country, particularly in the case of 

 schists, all traces of bed-rock striae have 

 seemingly been effaced by post-glacial 

 weathering. The country about Orange, a 

 little west of the north central part of Mas- 

 sachusetts, affords a good example of the 

 case in point. The rocks are soft gneisses 

 and hornblende schists. They strike nearly 

 north and south and dip about vertically, or, 

 in other words, stand on edge. Their very 

 attitude, combined with the local variation 

 in mineralogical composition and texture, 

 due to the banding in the gneiss, has en- 

 abled the process of weathering to work at 

 its maximum rate. As a result, the sur- 

 face of the rock, wherever exposed, is cor- 

 roded to extreme roughness, and often lon- 

 gitudinally pitted, so that on the rock it- 

 self about all trace of striie has vanished. 

 Also the approximate coincidence of direc- 

 tion between the stri;e and the strike or 

 banding in the gneiss renders any trace of 

 weathered stride which may remain not 

 only difficult of detection, but unsatisfactory 

 to the geologist, even when found. 



There is, however, a means of determin- 

 ing the direction of ice-movement in this 

 region. Happily the rocks are traversed 

 here and there by quartz veins of moderate 

 size. These veins being more resistant, of- 

 ten stand out in bold relief above the en- 

 closing rocks now weathered down at their 

 sides. They have retained not only their 

 ice-polished surface, but this surface is of- 

 ten found to be well marked by sharply de- 

 fined striae and very fine parallel scratches, 

 concerning whose origin the lens leaves no 

 doubt. 



These scratches sometimes occur in such 

 delicacy as to render detection by the 

 unaided eye difficult in ordinary light. By 

 chance it was observed that in reflected 



sunlight the most delicate become readily 

 visible, even at several yards distant. The 

 distinctness with which the striae are 

 brought out is due to the marked contrast 

 produced by difference of reflection between 

 the unstriated part of the ice-polished sur- • 

 face, which strongly reflects the light, and 

 the striae themselves, which do not reflect, 

 but appear as opaque or dark lines in a 

 bright shining background. 



Further observation seems to show that 

 this means of detecting striae can in many 

 cases be used to advantage, especially where 

 the surface to be examined is of considera- 

 ble extent, the task of observation being 

 materially facilitated without impairment 

 of reliability. The strife show best when 

 observed in the direction of their drift 

 trend, and with the angle of reflection large, 

 forty-five or more degrees. 



The above observations were made early 

 in April in connection with a visit to Mount 

 Monadnock, in New Hampshire ; a covering 

 of snow and ice preventing the taking of 

 similar observations on the mountain at the 

 time. It has since been learned, however, 

 from Mr. C. L. Whittle, who has made a 

 specialty of ice-movement over this moun- 

 tain, that, as in the region of Orange, the 

 striae are now chiefly limited to the exposed 

 edges of quartz veins traversing the gran- 

 itic gneisses and other rocks which consti- 

 tute the mountain. F. C. Scheadee. 



CAnrBEiDGE, Mass., May 2, 1896. 



OCCUBBENCE OF UINTAITE IN UTAH.* 

 The name Uintaite was given to the hard 

 asphaltic substance to be discussed, by 

 Prof. W. P. Blake in 1885. Subsequently 

 it acquired the name Gilsonite, after -a Mr. 

 S. W. Gilson, of Salt Lake. 



In appearance Uintaite is jet black, of 



* Read by Mr. George H. Eldridge before the Ge- 

 ological Society of AVashington, January 8, 1896, and 

 reported with the author's approval by Dr. W. F. 

 Morsell. 



