SIB 



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and protogine, and also among the 

 schistose rocks. The following 

 species are enumerated : granular ; 

 large grained; fine grained; gra- 

 nitic; quartzose; compact; zoned; 

 agate-like ; porphyritic. They all 

 occur in Cornwall. 



SI'BERITE. Another name for Rubel- 

 lite, or red tourmaline. 



SI'DERO-CA'LCITE. The name given by 

 Kir wan to brown spar ; the braun 

 spath of Werner. 



SI'ENITE. ) (from Syene, a city of 



SY'ENITE. ) Egypt, where this rock 

 occurs in abundance, and whence 

 the Eomans obtained it for archi- 

 tectural and other purposes. ) Wer- 

 ner gave the name of sienite to 

 aggregates composed of felspar, 

 hornblende, and quartz ; or of fel- 

 spar, hornblende, quartz, and mica. 

 Sienite is the roche feldspathique 

 of Haiiy. It often bears the ge- 

 neral aspect of a granite. Felspar 

 and hornblende may be deemed its 

 two constant and essential ingre- 

 dients, but it frequently contains 

 quartz and mica, and occasionally 

 talc and epidote. It is the presence 

 of hornblende, as a constituent 

 part, which distinguishes this rock 

 from certain granites, that acci- 

 dentally contain hornblende. The 

 structure of sienite is sometimes 

 slaty, commonly granular. Green- 

 stone and sienite are essentially 

 composed of the same ingredients, 

 namely, felspar and hornblende ; 

 from granitic greenstone there is a 

 transition to sienite, and from sie- 

 nite to true granite. The colour 

 of sienite is usually grey, but this 

 is affected by the ingredients en- 

 tering into its composition. Prof. 

 Delesse says, "the rose-coloured 

 syenite of Egypt is formed of 

 quartz, orthose, oligoclase, mica, 

 and frequently also of hornblende." 



SIEXI'TIC. Containing sienite; re- 

 sembling sienite; possessing some 

 of the characters of sienite. Sie- 

 nitic granite contains hornblende. 



Sienitic porphyry is fine-grained 

 sienite containing large crystals of 

 felspar. 



SIGAEE'TTJS. A genus of marine uni- 

 valve shells belonging to the family 

 Macrostomata. It is a depressed, 

 oval, nearly auriform shell, with a 

 short spiral columella : the aper- 

 ture entire, wide, spread out to- 

 wards the summit of the right lip, 

 and longer than wide. It is a 

 Tuscan fossil, and exceedingly rare. 

 The living sigaretus is found in 

 sand at depths varying from five to 

 fifteen fathoms. 



SIGILLA'BIA (from aigillum, Lai) 

 The name given by Brongniart, to 

 certain large, and, in modern vege- 

 tation, unknown forms of plants 

 discovered in the coal formation: 

 the name has been assigned from 

 the peculiar impressions on the 

 stems. The stems are of various 

 sizes from a few inch s to upwards 

 of three feet in circumference, and 

 of great length. A stem found in 

 Craigleith Quarry, near Edinburgh, 

 was forty-seven feet in length, the 

 bark being converted into coal. 

 Stems nearly as long, .and four feet 

 and a half in diameter, have some- 

 times been found in the coal series 

 in the north of England. They are 

 scattered throughout the sandstones 

 and shales that accompany the coal, 

 and may occasionally be seen in the 

 coal itself. These stems are in- 

 clined in all directions, and some 

 of them are nearly vertical : they 

 are supposed to have been hollow, 

 like the reed, and with but little 

 substance. Brongniart has enume- 

 rated nearly fifty species. 

 SI'LEX. (silex, Lat. flint.) An oxide 

 of silicon, constituting the greater 

 part of all the rocks of which the crust 

 of the earth is composed. Pure silex 

 is perfectly white, it has neither 

 taste nor smell, and its spec. gr. is 

 2'26. Silex consists of oxygen 

 in the proportion of 50 per cent, 

 united with the base silicium ; it 



