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I L 



a bed or stratum ; as the whinstone 

 sill. The great bed of basalt in 

 Northumberland is called the 

 Whinstone Sill. 



SI'LLIMANITE. A dark grey or brown 

 mineral, composed of silica 42'6, 

 alumina 54'1, oxide of iron 1*9, 

 water 0-5, discovered at Say brook, 

 in Connecticut, and named after 

 Prof. Sillirnan. 



SILT. The deposit of running water ; 

 mud. 



SILT. To silt up, to fill with mud, 

 sand, or other matter deposited by 

 running water. The verb is com- 

 monly used with the word up im- 

 mediately following, as the silting 

 up of rivers ; estuaries known to 

 have been silted up, &c. 



SI'LVAN. The name given by Werner 

 to the metal tellurium. 



SI'LVEE. (silber, Germ.) One of the 

 simple or elementary bodies, and 

 included in the subdivision termed 

 metals. When pure, it is nearly 

 white. It is superior to gold in 

 lustre, and inferior to it in malle- 

 ability ; it is however so malleable 

 that it may be beaten out into 

 leaves not exceeding 100,000th of 

 an inch in thickness. It is very 

 ductile, surpassing gold in tenacity, 

 but inferior to iron, copper, and 

 platinum. It may be drawn out 

 into wire of greater fineness than 

 human hair. It is harder than 

 gold, but softer than copper, and 

 may be easily cut by a knife. Its 

 specific gravity is 10'47. Silver 

 fuses at a temperature of about 

 1,000 degrees Fahr. It is not 

 oxidated by exposure to the atmos- 

 phere, but becomes tarnished by 

 sulphureous vapours. It is taste- 

 less, and free from smell. It is 

 soluble in nitric acid. Silver for 

 domestic purposes, as well as that 

 made into coin, is rendered harder 

 by an alloy of copper. The stand- 

 ard silver of this country consists 

 of eleven ounces two pennyweights 

 of pure silver and eighteen penny- 



weights of copper. Silver has been 

 known from the earliest ages. It 

 is found native and in ores of 

 several kinds. The ores of silver 

 occur in metallic veins, traversing 

 primary rocks. There are many 

 ores which yield silver that are not, 

 strictly speaking, ores of silver. 

 Although the mines of Europe yield 

 considerable quantities of silver, 

 yet it is to Mexico and Peru that 

 we are indebted for the main sup- 

 plies. The mines of Potosi have 

 paid a royal duty on silver valued 

 at 234,700,000 sterling. 

 SILU'EIAN. The name given by Sir 

 R. Murchison to an upper subdi- 

 vision of the sedimentary strata 

 found below the old red sandstone. 

 He assigned this name to these 

 strata from their being best de- 

 veloped in that portion of England 

 and Wales formerly included in the 

 ancient British kingdom of the 

 Silures. The Silurian rocks are 

 divided into upper and lower : the 

 upper Silurian rocks comprise the 

 Ludlow formation and the Wenlock 

 formation ; the Ludlow formation 

 consisting of the upper Ludlow 

 rock, the Aymestry limestone, and 

 the lower Ludlow rock ; the Wen- 

 lock formation consisting of the 

 Wenlock limestone and the Wen- 

 lock shale : the lower Silurian 

 rocks are subdivided into the 

 Caradoc formation and the Llan- 

 deilo formation ; the Caradoc for- 

 mation consisting of flags, sand- 

 stones, grits, and limestones ; the 

 Llandeilo formation, of calcareous, 

 dark-coloured flags, with sandstone 

 and schist. The whole of the 

 Silurian rocks attain a thickness 

 in some parts of seven thousand 

 five hundred feet. They are all 

 marine deposites. The Silurian 

 rocks, though ancient, are not the 

 most ancient of the fossiliferous 

 strata. They are but the upper 

 portion of a succession of early 

 deposits, which it may hereafter be 



