CYRSTOSPHENES OR BURIED SHEETS OF ICE 235 



to one or two hundred feet, as shown by shafts sunk through them 

 at various places. 



Speaking generally, these ice- sheets are of very even and regular 

 thickness throughout, though they are not strictly horizontal, but 

 approximate closely to the slope of the surface under which they 

 lie. For instance, the city of Dawson is built on an alluvial bottom 

 land declining gently from the base of a steep hill to the banks of 

 the Yukon and Klondyke Rivers, and a crystosphene which here 

 underlies the surface at a few feet beneath it seems to have about 

 the same slope. In another case a crystosphene was encountered 

 on a mining claim on Hunker Creek three feet below the surface, 

 and it was traced for five or six hundred feet down the valley, 

 being everywhere at practically the same depth, while the surface 

 itself had a slope of about one in a hundred, so that this apparently 

 level sheet .of clear ice was five or six feet higher at its upper end 

 than at its lower. Examples of this kind could be multiplied almost 

 indefinitely, showing plainly that these ice-sheets do not partake 

 of the character and attitude of frozen ponds or lakes. 



While these crystosphenes, or so-called "glaciers," are usually of 

 the nature of nearly horizontal sheets, occasionally they occur as 

 veins or dikes of ice rising through the bed-rock into the overlying 

 gravel. Two such veins of ice were very well exposed in the under- 

 ground workings on mining claim No. 39, below Discovery on Hunter 

 Creek, where they evidently represented the former course of a 

 spring, which had changed its point of discharge. More or less ver- 

 tical masses of ice are also sometimes met with in the gravels them- 

 selves, indicating the positions of former water channels from the 

 bed-rock toward the surface. 



In the majority of cases crystosphenes are in the vicinity of springs 

 that can be plainly seen issuing from the bases of the neighboring 

 hills, but in other cases no such springs are apparent. In these 

 latter cases, however, wherever the gravel has been removed, and 

 the underlying rock has been exposed, springs have been found. 

 While studying the origin of the crystosphene 600 feet long, already 

 mentioned as occurring on Hunker Creek, no springs were apparent 

 in the immediate vicinity, and at first it seemed as if the ice must 

 have been formed from water flowing from a spring three or four 

 hundred yards farther up the valley; but finally a httle trickling 



