G N E 



G N E 



of the worms bv the authors who have trcaU-d of Uu-m ; 

 which may ah'o be not a httle added to by the particular 

 views in which thofe dn.ughts have been taken. ^ Notwith- 

 ftanding al! the variations of thcfe figures, the general form 

 of the animal is however the fame in all, and the fpecies can- 

 not but be known from any of them. ^V'ilik■ the v.orm is 

 vDung, the body is whilidi or grecnilh ; but when it is at its 

 full growth, and draws near the time of its changes, it be- 

 comes greyifh. The great tranfparence of the body of this 

 worm gives a fine view of what paffes within it ;'it is at any 

 time eafy to fee into the motion of the intellines, by wliich 

 the food is puflied on towards the anus. The two princi- 

 pal trachea; are alfo fccn very dilliiicily in this creature ; they 

 arc two white tidies jdaced in a parallel direftion one to 

 another, and run from the firft ring to the tube of refpiration. 

 The (;reat worm feveral times ciiangcs its flcin in the courle 

 of its life. After three changes of tliis kind, which ufualiy 

 happen in tlie fpace of three weeks, or thereabout, it under- 

 goes a fourth, wliere the old {Icin is as eafdy thrown off as 

 in the reft, but. the animal now appears in anew form, that 

 of a nymph ; it is now fliorter and rounder than before, and 

 the botly. is fo bent that the tail is applied to the under part 

 of the head ; this, however, is only its form in a voluntary 

 ftate of reft, for it can yet move, and when it pleafe-s extends 

 its tail, arid fwinis abot;t as fwiflly as when in its other itate. 

 When the creatures have quitted tiieir firtl habitations and 

 the figure of worms, they re-afcend to the top of the water, 

 inclofed in a kind of ftiell, witli a large head and moulli, two 

 black eyes, two horns, feveral tufts ot hairs on .ditierent 

 parts of the body, and a tail with a brulh of Irair at the end 

 of it, which, being fmeared over with an oily fluid, ferves 

 to keep them above water ; their heads benig fometinies 

 lifted in the air, and fometimes plunged into the water, while 

 the tail Hides along the furface : and when the oil on the tail 

 begins to dry, they fhed from their mouth a frefli fupply, 

 vvliich renders it capable of fteering where they pleale, with- 

 out being wetted and damaged by the water. 



All the parts of the futm-e gnat may be feen in this 

 nymph; the (Icinof it is extremely thin and Iranfparent, yet 

 fufficiently tough and firm for the ufe it is intended for. It 

 is uncertain how long exaftly the animal lives in this 

 nymph ftate ; but after the time is accompliflied, its 

 change into a gnat is very quick, and is attended with 

 great danger to the animal, iince multitudes are drowned 

 in the aiil of getting out and fpringing into the air. 

 Reaumur, Hift. Inf. vol, iv. Baker's Microf. 1743. p. 88. 

 Gn"AT, in Ruriil Economy., a fmall well-known infeft, 

 ■which is faid to deftroy the leaves of fome tender vegetables 

 as foon as they appear, fuch as the turnip, &c. 



GNEISS, in Min.ral'.gy, a primitive rock, compofed of 

 fe'dfpar, quartz, and mica; incredients'whichare likewifethofe 

 of granite, from which it diitcrs in the arrangement of thofe 

 parts ; for while in the latter rock they ufualiy appear as gra- 

 nular aggregations, thofe of gneifs arc difpofedin luch a man- 

 ner as to exhibit a more or lels flaty ttructure. This ftrudlure 

 paffes through various degrees of diftinftnefs ; on one hand 

 it approaches fo near to the granular texture, that the diilinc- 

 -tion between gneifs and granite ceafes to exift, while, on the 

 other hand, when its peculiar texture is very obvious, and 

 becomes what is termed thlnjlaty, a paftage is formed into 

 tite primitive rock, next to it in antiquity, called mica-flate. 

 In its progrefs towards the nature of granite, the feldfpar 

 is generally predominant with regard to the mica ; while 

 thofe vai^ieties approaching mica-flate gradually exhibit a 

 fmaller proportion of feldfpar and much mica. Sometimes, 

 however, the mica is only apparently predominating, owing 

 to the circumftance, that on breaking a mafs of gneifs, the 



i 



line of feparalion will more frequently pafs through the 

 mica than through the other layers, and difplay a whole fur- 

 face of that fubftance ; but anotlier frafture, perpendicidar to 

 the firl't, will, in this cafe, foon undeceive the obferver. 

 Though feldfpar is, generally fpeaking, the predominant 

 ingredient of gneifs, yet the proportion of mica in this rock 

 is, upon the whole, greater than what we fee it in granite : 

 and tl;e lefs this is the cafe, the more the mafs appears granu- 

 lar, and the more it approaches to granite. It is often a 

 matter of diiliculty, in viewing fmall ipccimens only, to dif- 

 tinguilli fome kinds of gneifs from the laft-mentioned primi- 

 tive rock ; when the infiieftion of tlie former in their natural 

 fituation, and en a large Icale, would have left but little 

 doubt in the mind of the examiner. ' - 



Gneifs, viewed in the large, as a mountain mafs, always 

 exhibits thick and diftiiK-i ft rata, whofe outgoings, or upper 

 terminations, are generally lower than the fubjacent granite, 

 and higher than thofe of the lupcrincumbent mica-flate. 



Werner (to whom we ewe more correct ideas refpeftingthis 

 rock,) diftinguiflies four kindsof gneifs, -viz. that approaching 

 the granular ftruiture, the waved or undulated, the common, 

 and the thin ilaty gneifs. As particular varieties of thefe, we 

 have I. The ftriped gneifs, in which the quai-tz is difpofed in 

 narrow bars, furroimded by feldfpar, producing, on its longi- 

 tudinal fracture, a ftriped appearance, not unlike that of lome 

 kinds of petrified wood ; wla'e its tranfverfal fratlure exhibits 

 a granular difpofition of the parts. This ftriped variety gene- 

 rally occurs together with the waved ; fuch as at Ober-New- 

 flionberg, Reifland and Hartmanfdorf, near the BchemiaiJ 

 frontiers. 2. The fprinkled, or that kind of gneifs in which 

 the feldfpar and mica exhibit themfelves in the form of fmall 

 nefts : fuch as that found at Hartmanfdorf and Bobritfch, 

 near Freiberg, and at Kuffenberg in Bohemia. 3. The 

 fiiort lamellar or flaky variety, fuch as it is found at Marien- 



berg 



'V\\s colour of the ingredients of gneifs is not fubjeft to 

 great variation. The feldfpar. is generally of a greyiih, yel- 

 lowifti, and reddifh-white colour, feldom yellowifti-grey, or 

 deep fiefli-red, as it appears in many varieties of granite ; 

 upon the whole, its tint rcfembles that of the quartz with 

 which it is accompanied, and which feldom appears fmoke- 

 grey or brown. The colour of the mica is generally black 

 or brownifli-black, fometimes browniih, yellowilh, and alh- 

 grey, and rarely iilver-white or of a golden hue. 



Thefe component parts are found to vary alfo in regard 

 to their frc/l.'ncfs ; in general the feldlpar is perfeiJtly fo- 

 liated and ftiining ; but iometimes it appears in incipient de- 

 compofition, or even converted into petunze ; in the fame 

 manner as the mica is now and then feen paffing into a grecnift 

 fubftance rcfembling fteatite. Dr. Reufs found that the' 

 porcelain earth dug at the Galgenberg, near Pufchwitz,'in 

 the Saatz diftrift of Bohemia, v.here it is ufed for white- 

 wafliing walls, is tlie refult of the higheft ftage of decompo- 

 fition of gneifs ; it forms thick ftrata, and contains a great 

 quantity of fmall grej-ifti white, light grey, and pale clove- 

 brown grains of quartz. In the lame manner this minera- 

 logift obfervcd in that country frequent preofs of the con- 

 verfion of the feldfpar of gneifs into a ferruginous clayey 

 mafs. Near Klofterly it is feen converted into bunt-tlioo 

 or variegated clay. 



Befides the more effintial feldfpar, quartz, and mica, the 

 following accidental ingirjirnts are now and then met witli in 

 gneifs. i.Shorl: both the common and black tourmaline 

 (horl, fometimes occur in the gneifs of I'reiberg, and parti- 

 cularly of Spain. 2. Garnet; rarely, but much more frequently 

 than in granite; it occurs at Wiefenthal- in Saxony, in Nori. 

 way, in the illand of Zealand, in Moravia, im. 3. •Horn- 

 blende 



