S I E 



S 1 E 



jfranite, intermixed witli a fmall quantity of hornblende. 

 Some fienites are coarlely granular, containing large dif- 

 tintt cryftals of red felfpar ; in other fienites the cryftals 

 are very minute, and the hornblende greatly predominates : 

 in the latter cafe, fienite paffes into the rock which the 

 Germans call grun-ftein, or green-ftone. When the fienite 

 is finely granular, but contains large cryftals of felfpar im- 

 bedded, it is then denominated fienite porphyry. This 

 rock frequently occurs in large continuous malles, lying 

 over granite and (late rocks. It is fometimes divided into 

 diltinft layers, which, by the German geologifts, have been 

 called ftrata ; in other inltances, it adumes the prifmatic or 

 columnar form. Some mineralogifts clafs all thofe granites 

 with fienite, which contain a portion of hornblende ; but 

 this diftinflion appears ufelcfs, for the fame mineralogifts 

 admit that hornblende is frequently a conllituent part of 

 gneifs or flaty granite. 



In the infancy of fcience, fubftances which are eftentially 

 diftindt, are apt to be confounded together, if they pollefs 

 certain points of refemblance ; and in a fomewhat more ad- 

 vanced ftate of information, fubftances, that are eftentially 

 the lame, are often feparated, by artificial claffifications, 

 into diftinft orders or Ipccies, by thofe who are accuftomed 

 to infpeCt the cabinets of colleftors, and overlook the great 

 features of nature. Thus much ulelefs difcuflion has arifen 

 with refpeft to certain rocks, whether they were to be 

 clafled with granite or fienite, S:c. The operations of na- 

 ture, in the compofition of the great rock formations, are 

 not to be limited by the definitions of the mineralogift. 

 Almoft all rocks, with the exception of the calcareous, pafs 

 by fuch infenfible gradations into each other, that the moft 

 experienced eye would find it difficult to clafs them, from 

 the infpeftion of detached fpecimcns. Nor will tliis appear 

 furprifing, when we confider that filcx generally compofes 

 half or more of the fubftance of all rocks not calcareou", 

 whether fimplcor compound ; and even the diltinft concre- 

 tions in compound rocks generally contain about the fame 

 proportion of filex, except quartz, which is almoft entirely 

 formed of it. Even in common clay, though the properties 

 of filex are concealed by a portion of alnmine, yet it is llill 

 the predominating part of tiie mafs. Now whether rocks 

 vere confolidated from a folution, or from a ftate of igneous 

 fnfion, the filex, or principal ingredient, as it became va- 

 rioiifly intermixed with the other earths, would, on confoli- 

 dation, pafs into a vaft variety of forms. Where the 

 folution or fufion continued for a longer time uiidilturbed, 

 the laws of chemical affinity would have freer aftion, ai;d 

 difpofe the different elementary parts to affume thofe dif- 

 tinCt cryftalline forms peculiar to certain combinations and 

 proportions of the earths. Wliere, from change of tem- 

 perature, or other caufes, the play of chemical affinity was 

 interrupted, the mafs would become confufedly cryftalline, 

 and lofe all appearance of cryftallization ; and between thefe 

 extremes there might be every poffiblc variety of gr.idation, 

 fuch as we now find to exift. 



Hornblende appears to be the fubftance which connefts 

 granite with rocks that have a very diflerent external ap- 

 pearance. Wiicn hornblende exifts in (mall quantities in 

 granite, it does not change the character of the rock, where 

 the threee eftential fubftance", quartz, fellpar, and mica, 

 form the prevailing part of the mafs ; for chlorite, fteatite, 

 and other minerals, arc intermixed in fmall quantities in the 

 granite of the higher Alp''. When the quantity of horn- 

 blende is incrcafed, and that of the mica and quartz di- 

 minifhed, the rock is properly what mineralogifts deno- 

 minate fienite. When the hornblende and other minerals, 

 except felfpar, are fo irtimately mixed as to form one 



homogeneous palle, in which the cryftals of felfpar are im- 

 bedded, it then conftitutes a porphyry. When the horn- 

 blende greatly predominates, and the felfpar is in very fmall 

 cryftals, it forms green-fti)ne. When the cryftals are no 

 longer difcernible, it forms rocks to which the name of trap 

 have been given. (SeeTnAr.) Green-ftone may be con- 

 fidercd as a cryftalline variety, and bafalt as an earthy 

 variety, of trap. Thus by an intermixture of hornblende, 

 we have an infenfible gradation from highly cryftalline gra- 

 nite and fienite to a compaft earthy rock, in the internal 

 ftrufture of which fcarcely any trace of cryftallizatioa can 

 be feen. 



Sienite is found in Cornwall, and the weftcrn fide of 

 Wales, and at Chamwood fjreft in Lciceftcrftiire : from the 

 latter place it is fent to London, and to Nottingham and 

 Leicefter, cut into blocks for paving-fto'.es. A beautiful 

 green and red fienite forms one of the hills called Mark- 

 field, which is defcribed in Mr. Bakeweli's Geology, as 

 covered on its fummit with pyramidal blocks of the fame 

 ftone, which he conceives miglit be emphiyed for durable 

 ornamental architefture. Sienite occurs in many of the 

 mountainous parts of Scotland, but is not, we believe, a 

 metalliferous rock in any part of Great Britain ; nor has it, 

 we believe, been applied to any other purpofe than for 

 paving-ftones. Many of the ancient ftatues and columns in 

 Egypt are formed of this ftone ; and it has been before ob- 

 ferved, that it was brought in great quantities from that 

 country by the Romans, on account of the vaft columns, &c. 

 which could be formed from fint'le blocks. As a proof of 

 its durability, it is ftated by travellers that colum.ns whicii the 

 Romans left unfiniftied in the quarries of Sienna, nearly 

 two tliouland years fince, preferve to the prefent day the 

 impreffion of the tools as (harp and diftinft as if they had 

 been recently worked. 



SIEN-KU, in Geography, a town of China, of the third 

 rank, in Tche-kiang ; i8 miles W.R.W. of Tai-tcheou. 



SIENNA, or SiEN'A, a city of Italy, and capital of a 

 diftridf, in Etruria. This diftrift, called Siennefe, or Ter- 

 ritorio di Sienna, was once a free republic, conquered by 

 the emperor Charles V. in the year 1554; whofe fon, 

 Philip II. king of Spain, ceded all parts, except the Statu 

 de gli Priefidii, to Cofmo I. duke of Florence. The city 

 is pleafantly built on three hills, in a healthy fituation. 

 The houfes are of brick, and the ilrects paved with the 

 fame. It is nearly five miles in circumference, but not 

 peopled in proportion to its fize ; the inhabitants fcarcely 

 exceeding 17,000. It is the fee of an arclibifliop, and the 

 cathedral is a fine Gothic building, coated with white and 

 black m.arble within and without. The great portal was 

 begun in the year 1284, after the defigns of Giovanni da 

 Pila, and finifticd in 1333, by Auguftino and Agnolo, 

 Siennefe architefts. The (ront is prodit^ioufly loaded with 

 ornaments. All the work of the iiifide is moft highly 

 finiflied, as the carving in wood of the choir ; the fculpture 

 in marble of the pulpit ; and efpecially the liiitorical en- 

 graving of the pavement, reprefenting in chiam feuro the 

 moft remarkable llories of the Old and New Teftamcnt. 

 From an aperture in the choir pavement may be (een St. 

 John's church, whicli lies dirertly underneath. Its entrance 

 is without, on the hill ; and thus one chureli may be laid to 

 ftand oil another. Near the cathedral is the archhilliop's 

 palace, and oppofitc to it a large ami well-endowed liolpitui, 

 founded by a (lioc-maker. Mere are alio many other 

 churches and convents. The church of the Dominicans is 

 remarkable (or an ancient pidiire of wood, reprefenting the 

 Virgin with the infant Jefiis in lirr arms, by Guido Sancfe : 

 it is dated 1221, and is in the Venturini chapel: though fo 

 ^. Q 2 ancient, 



