778 GEOLOGT. 



successively occupied by water; but whether that of lakes which have vanished through 

 the bursting of some barrier, or of rivers which have cut their way down to a lower site 

 through diluvial sediments, and lost part of their volume ; or whether there has been 

 a displacement of the land by elevation, and no actual subsidence of water, are points 

 which remain open to discussion. The most striking example of these terrace construc 

 tions is known as the "parallel roads of Glen Roy," so called from their perfect re 

 gularity, and resemblance to artificial embankments. Glen Roy is a highland valley in 

 the district of Lochaber, divided into upper and lower, the Upper division being about 

 four miles in length by one or more in breadth, bounded on the opposite sides by high 

 mountains, from which two streams descend, and join their currents towards the centre. 

 The united water forming the Roy then flows gradually to a rocky pass, through which 

 it proceeds in a troubled course, and emerges into the wide, long, and sinuous valley of 

 lower Glen Roy, where the terraces or parallel roads occur. Sir T. D. Lauder and 

 Dr. Macculloch ascribe the terraces to the operations of a lake whose waters were suc 

 cessively lowered by the removal of obstructions from time to time, now represented by 

 the stream that flows through the glen. 



5. Chemical and Mineral rock-salt ; calcareous tufa or travertine ; siliceous sinter ; 

 bitumen. 



Chloride of sodium, or rock-salt, is deposited from solution in the waters of the ocean, 

 of lakes, and springs ; but the formation is rarely visible from the circumstances under 

 which it transpires, though at the bottom of such seas as the Mediterranean the depo 

 sition is supposed to be proceeding extensively, and in some of the cavities of the rocks 

 along the shores it accumulates in such quantities as to be collected by the inhabitants. 

 The beds of the great salt lakes of Persia and Asia Minor are also composed of saline 

 incrustations, with which the shores appear coated during the annual reduction of the 

 water to a lower level in the summer season ; and table salt is largely obtained from 

 saline springs in various countries by the artificial evaporation of the water. These 

 springs usually contain a greater proportion of salt than the waters of the ocean, those 

 of Cheshire yielding 25 per cent., and of the United States from 10 to 20 per cent., 

 whereas sea water rarely contains more than 4 per cent. In the year 1829 the quantity 

 of salt made from the springs in the United States amounted to 3,804,229 bushels ; but 

 in 1835, from the Onondago spi-ings in New York alone, 2,222,694 bushels were obtained, 

 and 3,000,000 bushels from the Kenhaira springs in Virginia, the borings in some places 

 extending to the depth of a thousand feet. 



Calcareous tufa or travertin, a deposit of carbonate of lime from springs and rivers 

 holding the substance in solution, is a light, porous, vesicular mass when first produced, 

 as the word tufa implies, but forms a solid limestone, sometimes even crystalline, upon 

 exposure to the atmosphere, so as to be used for architectural purposes. The thermal 

 waters of Central France, Hungary, Tuscany, and Campagna di Roma, yield it abun 

 dantly ; but it is also deposited by springs of the ordinary temperature, and is precipitated 

 by the Tuscan rivers as well as those of Asia Minor. In the vicinity of Rome some of 

 the calcareous tufa cannot easily be distinguished from statuary marble ; and that which 

 is constantly forming near Tabreez, in Persia, is a beautiful variety of semi-transparent 

 marble or alabaster. The concretionary calcareous deposits formed in caverns, the sta 

 lactites depending from the roof, and the stalagmites resting on the floor, are similar pro 

 ductions. Siliceous sinter is an aggregation of successive films or scales of silica {sinter, 

 a scale), formed from the waters of thermal or hot springs, which sometimes hold that 

 earth in solution. The chief sites of its production are at the Geysers in Iceland, where 

 a siliceous deposit, nearly a mile in diameter and twelve feet thick, occurs ; and in the 

 Azores, where elevations of siliceous matter are found thirty feet high. In these dis- 



