Geology and Mineralogy. 313 



the glacial theory of the origin of the Parallel Roads of Glenroy. 

 The Jacobshavn glacier stops up both ends of a valley running 

 parallel with its course, converting it into a lake which is sepa- 

 rated from the glacier throughout the greater part of its length by 

 a " Nunatak." The lower end of another valley considerably to 

 the south of this is stopped by the ice-sheet, and the valley con- 

 verted into a lake (Tasersiak), which is drained by a river 

 running over the col at the head of the valley into the Stromfjord, 

 just as in the case of the Marjelen See, only the Greenland lake is 

 much larger than this. A similar lake drains into the N. Isortok 

 fjord, and another into that of Alangordlia. Two similar lakes 

 are formed to the east of Sermilik fjord, and several small ones 

 to the east of Bjornesund. North of the Frederikshaab glacier 

 is a valley running north and south, the mouth of which is 

 stopped by the Frederikshaab glacier, whilst a tongue of ice flows 

 through a col situated half-way up the valley and bars the valley, 

 one part of the tongue of ice flowing a small distance to the 

 north, and another to the south, thus causing the conversion of 

 the valley into two lakes. On the east of the Frederikshaab 

 glacier is the lake Tasersuak, bounded on the north by the 

 " Nunatak " Kangarsuk, and stopped at its lower end by the 

 Frederikshaab glacier, and having a tongue of the ice-sheet enter- 

 ing into it at the upper end. 



The erosive power of an ice-sheet is well seen by a glance at 

 the observations made upon the rivers which flow into the fjords 

 of Nagsugtok and Isortok, and which have their origin at the 

 ends of the tongues of ice which occupy the valleys continuous 

 with these fjords. The river from the first contained only 200- 

 225 grams of mud per cubic meter of water in the month of 

 July, while the second in the month of June enclosed 9129 to 

 9744 grams. This is compared with the amount carried by 

 the Aar where it emerges from the glacier ; it there contains only 

 142 grams. The great difference presented by the rivers which 

 fall into the two fjords is attributed to the fact that the ice moves 

 with much greater speed toward the fjord of Isortok than 

 toward that of Nagsugtok. It is calculated that the quantity of 

 fine mud carried into the former of these fjords amounts to 4062 

 million kilograms per day. This mud is deposited in the 

 interior of the fjord, which is filled up to such an extent in its 

 upper portion that even flat boats cannot pass up it. What a 

 powerful machine for the formation of the fine clay of ' ; till " 

 is here. 



2 Variations in water-level in enclosed Seas.- — Professor 

 Suess has recently discussed the phenomenon of variation in the 

 relative heights of water and land, more especially for the Baltic 

 Sea, and with reference to the different hypotheses that have been 

 advanced to explain it. He decides in favor of the opinion held 

 by Nordenankar, that the phenomenon is not due to elevation of 

 the land but is essentially one of variations in the height of the 

 water as caused by the varying precipitation and outflow. The 



