769 



SILICULA. 



SILURIAN SYSTEM. 



770 



are silicates of alumina and lithia. Epidolc and Idocraie are silicates 

 of alumina, lime, and iron. Garnet ia a silicate of alumina, iron, and 

 manganese. 



Tourmaline is a compound of silica, alumina, lime, iron, soda, and 

 boracic acid. Axinite is the same, with magnesia, instead of lime. 

 Jolite is a silicate of alumina, magnesia, iron, and manganese. 



Mica, consists of silica, alumina, potash, iron, fluoric-acid, and 

 water. The Topaz is a silicate and fluoi-ide of alumina. Lapia 

 Lazuli is a silicate and sulphate of alumina, soda, lime, and iron. 

 Sodalite is a silicate of alumina, with chloride of sodium. 



The Beryl, Enclose, and Chrytoberyl, consist of silica, alumina, and 

 glucina. The most important of these minerals are described under 

 their proper names in this work. 



SILI'CULA, a kind of Fruit. [FRUIT.] 



SI'LIQUA, a genus of Mollusca. [PYLORIDIA.] 



SI'LIQUA, a kind of Fruit. It is characterised by having one or 

 two cells, with many seeds, dehiscing by two valves, which separate 

 from a central portion called the Replum. It is linear in form, and is 

 always superior to the calyx and corolla. The seeds are attached to 

 two placentae, which adhere to the replum, and are opposite to the 

 lobes of the stigma. This position of the seeds, being abnormal, can 

 only be explained in two ways: either this fruit is in reality composed 

 of four carpels, two of which have, during the growth of the pistil, 

 become abortive ; or the stigmas must be looked upon as the fusion 

 of two halves, one from each side. The dissepiment of. the fruit in 

 this i-.iae is most probably a spurious one formed by the projecting 

 pine. to. It is sometimes found incomplete, from the edges of the 

 placentae not meeting ; it is then said to be ' fenestrate.' This kind of 

 seed-vessel is possessed by a large number of plants belonging to the 

 order Cruciferce, and examples may be seen in the Stock or Wall-Flower 

 -{Cheiranthut), ir the Ladies' Smock (Cardamine), and in the Cabbage, 

 Turnip, and Mustard. The Linmean class Tetradynamia is divided 

 into two order?, according to the form of its fruit : those plants of the 

 class having a silique are comprised under the order Siliijuosa ; those 

 having a silicle, under the order Siliculosa. 



SILK-COTTON. [BOMBAX.] 



SILK-WORM. [BOMBYCID.E.] 



SILLIMANITE, a Mineral a crystallised Silicate of Alumina. It 

 occurs in rhombic prisms imbedded in quartz. Cleavage parallel to 

 the long diagonal. Its colour is dark brownish-gray or clove-brown. 

 Fracture uneven, splintery. Specific gravity 3'41. Its lustre is 

 vitreous, nearly adamantine on the face of cleavage. Nearly opaque. 

 Hardness 8'0 to 8'5. It is brittle, and easily reduced to powder. 



It is met with at Saybrook, Connecticut, North America. It was 

 at one time considered to be a variety of Anthophyllitc, but it is much 

 harder than this mineral, aud contains more alumina and less silica 

 and oxide of iron. It more nearly resembles Sienitc, both in form 

 and composition. 



It yielded by the analysis of Bowen 



Silica . . 42-67 



Alumina ... . . .".I'll 



Oxide of Iron . . 2'00 



Water . . . 0'51 



99'29 



SILLOCK. [MERLANGBS.] 



SI'LPHIUM (2iA^ioy). Ancient authors mention this plant and its 

 juice. Two kinds of this substance are described : one, from Cyrene, 

 was probably yielded by Thapsia Silphium [LASER] ; and the other 

 was most likely assafoetida, which has been employed medicinally by 

 Asiatics from very early times, though it has been known by this 

 name in comparatively modern times. 



^iljihium was however remarkable for other properties, and hence 

 has attracted the attention of modern travellers who have recently 

 visited the countries where the Silphium is described as growing by 

 the ancients. The army of Alexander, in crossing the mountain 

 range which Arrian calls Caucasus (iii. 28, 10), aud which is the same 

 range that he afterwards mentions under the name of Faropamisus 

 (v. 5, 3), met with the Silphium. Arrian says, on the authority of 

 Aristobulus, "In this part of the Caucasus nothing grows except 

 pines and Silphium, but the country was populous and fed 

 many sheep and cattle, for the sheep are very fond of the 

 Silphium. If a sheep should perceive the Silphium from a dis- 

 tance, it runs to it, and feeds on the flower, and digs up the root 

 and eats that also. For this reason, in Cyrene, they drive the sheep 

 as far as possible from the spots where the Kilphium grows, and some 

 even fence in such places to prevent the sheep fiotn entering them if 

 they should approach; for the Silphium is worth a good deal to the 

 Cyrenaoans." Burnes, in crossing the Hindu Koosh, and seeing both 

 the men and cattle eating the young parts of the assafoctida plant, 

 supposed that it must be the Silphium of Arrian. But as this author 

 describes the country where the Silphium grows as abounding in cattle, 

 Dr. Royle had concluded that the Prangos of Mr. Moorcroft was the 

 Silphium alluded to, and which is much fed on by sheep and cattle in 

 the present day in Tibet. Mr. Vigne, when travelling in these regions, 

 came to the same conclusion. It is probable therefore that both 

 plant*, being umbelliferous, and employed for the same purposes in 

 nearly the same regions, may have contributed to form the accounts 

 which are so brief in ancient authors. [LASEK ; PHANUOS.] 



HAT. HIST. DIV. VOL. IV. 



SILURIAN SYSTEM. A considerable group of the Fossiliferous 

 Primary Strata, occurring in remarkable perfection in Wales, espe- 

 cially in the eastern and some of the southern districts, and in some 

 of the adjoining English counties, is thus named by Sir Roderick 

 Impey Murchison. 



When Sir Roderick commenced his researches in Shropshire and 

 Wales (1831), the principal knowledge we possessed of the succession 

 of the older stratified rocks of Great Britain, then commonly called 

 Grauwacke and Transition Formations, was based on the still incom- 

 pletely published labours of Sedgwick in Wales and the district of the 

 English lakes ; and so little was known of their fossil contents, that 

 it is believed that the first definite notice of this kind was contained 

 in Professor Phillips's description of a group of slate-rocks in the 

 vicinity of Kirby Lonsdale. ('Geol. Trans.,' 1827.) Now, inconse- 

 quence principally of the development given to this subject by the 

 appearance of the Silurian researches of Sir Roderick Murohison, aud 

 other works to which it has led, we are able to trace in one consecutive 

 history nearly the whole series of mineral depositions and organic com- 

 binations of which the ocean was anciently the theatre, from the period 

 of the Mica Schists to the termination of the Carboniferous era. 



In this survey, the Silurian Strata form a very conspicuous and 

 interesting portion, and in the district from which the type was 

 originally drawn they appear within distinct and definite limits, which 

 seem to insulate them from the older and new rocks, and to justify 

 their claim to the rank of a peculiar system ; but in other districts 

 phenomena appear which show that the order of physical changes and 

 organic combinations which characterise the Silurian System, was in 

 operation both before and after the period included in the ages of the 

 four Silurian groups of Llandeilo, Caradoc, Wenlock, and Ludlow ; 

 while in other districts these characteristic assemblages do not all 

 clearly appear; and thus we are naturally conducted to a more com- 

 prehensive view of the whole of the ancient (Palaeozoic) formations. 



Whatever be the true theory of the origin of the Granitoid Strata of 

 gneiss and mica schist (with their many and various quartzose, chloritic, 

 and calcareous accompaniments), it is at least certain, as a general rule, 

 that rocks of this general type are prevalent among the very deepest and 

 oldest deposits from water which retain proof of their watery aggrega- 

 tion, and that they are in this position devoid of the traces of ancient life. 



Equally certain is the character of the great series of Neptunian 

 rocks which lies upon the mica schist ; it is a vast and various mass 

 of strata (principally argillaceous, locally arenaceous or conglomeritic, 

 rarely yielding limestone), in which, though unequally, and in degrees 

 varying with locality, slaty cleavage tends to be developed. Organic 

 life has left traces in this series of muddy sediments both of vegetable 

 and animal origin ; in the lower and older parts very sparingly, in the 

 upper parts abundantly. If, with Professor Sedgwick and Sir Roderick 

 Murchison, we take the series of these rocks as they appear in Wales 

 and Cumberland, namely 



Silurian, or Upper group ; 

 Cambrian, or Middle group ; 

 Cumbrian, or Lower group; 



we shall find in the mineral characters of these groups, in the 

 countries named, some diagnostic marks of importance, but they 

 vanish or become equivocal in other regions. In like manner, the 

 organic contents seem, in the countries named, to be definitely 

 arranged in zones, so as to mark successive periods there. No organic 

 remains are known in the Cumbrian rocks; they are rare, and confined 

 to a few layers, in the Cambrian deposits; and are very plentiful and 

 general in the Silurian group. The districts in which these- pecu- 

 liarities occur are probably more wide and scattered farther asunder 

 than those in which the original types of mineral structure prevail ; 

 but yet it is evident that they are limited in respect of geographical 

 area, and variable in regard to the distinctness and completeness of 

 the terms, even in districts not far removed from the centre of investi- 

 gation. Let any one who may desire proof of this compare the 

 argil'.aceous series of Ayrshire, Westmoreland, Pembrokeshire, Tyrone, 

 or Waterford, in which Silurian fossils occur, with the full and varied 

 series of Shropshire, the Berwyn, and Snowdon. 



Under these circumstances of difficulty in regard to the right general 

 view of the ancient fossiliferous strata, we must consider the series 

 of Silurian rocks and fossils not as the type of this enormous sequence 

 of mineral and organic phenomena, but as one, and perhaps the richest, 

 of all the local physical combinations of that ancient period, and 

 employ it as a general term of comparison for reducing to order and 

 place many detached and difficult districts in which the strata have 

 local, peculiar, and perhaps exceptional aspects. 



Sir Roderick Murchison arranges the Silurian Strata in groups, as 

 follows, in a descending order : 



Formations. 



f 



Upper 

 Silurians 



Lower 



Silurians 



Ludlow rocks 



Wenlock rocks 



Divisions. 



( Upper Ludlow rocks 

 \ Aymestry limestone 

 ( Lower Ludlow rocks 



( Caradoc rocks . 

 ( Llandeilo rocks 



( \Venl 

 ( Wcnl 



lock limestone 

 lock shale . . 



Thickness 

 in feet. 



1500 



1000 



very 

 variable. 



Pyrogenous rocks are associated with the Silurian Strata in many 



3 D 



