§ 23. CONTORTIONS OF CRYSTALLINE ROCKS. 847 



the organism, as in other examples of silicified animal and 

 vegetable structures. 



Experiments have shown that melted quartz, unlike 

 alumina, retains its viscidity for some time when cooling, 

 and may be drawn out in threads ; * and M. Jobert states, 

 that in graphic granite he has found the quartz crystals in 

 the midst of the felspar flattened and contorted, as if they 

 had been strongly pressed between the felspathic matter. f 



These facts seem to offer an explanation of the flexures 

 and curvatures in quartz rocks and gneiss : and they are 

 brought forward by Mr. Darwin, with his usual acumen, 

 to illustrate the origin of the remarkable duplications, and 

 abruptly arched positions, of the stratified quartz rocks in 

 the Falkland Islands. " Some of the hills," he observes, 

 " are composed of quartz strata doubled on themselves, with 

 the axis-plane thrown quite over, — the quartz must there- 

 fore have been in a pasty condition when it suffered with- 

 out fracture such abrupt curvatures." Mr. Darwin states 

 that the detached concentric layers resembled gigantic 

 semi-cylinders of quartz, "like draining or ridge tiles. 



* M. Gaudin, quoted by Mr. Darwin. 



t The chalcedonic stalactites, which are often found in the hollows 

 of flint and agate nodules, must have been formed by the slow infil- 

 tration of silica in a viscid state. M. Alexandre Brongniart attributes 

 the formation of all agates and chalcedonies to the viscous or gelatinous 

 condition of the mineral matter ; and that of hyaline quartz and rock 

 crystal to the perfect fluidity of the same. " Lorsque la silice a ete 

 completement dissoute, et par consequent dans un etat de liquidite 

 parfaite, elle a cristallise et produit le quartz hyalin. Mais lorsqu'elle 

 etait en consistance gelatineuse, elle a produit les silex, et surtout ceux 

 qu'on designe par le nom general d'agate et de calc§doine." — Essai 

 sur les Orbicules siliceux. Ann. Sciences Xat. Juin 1831. 



In the specimen of Trigonia from Tisbury, previously mentioned 

 (ante, p. 520), the branchice are completely silicified; and some of 

 these processes when examined under a high power, show the orbicular 

 structure which characterizes chalcedonic silica. 



