298 EXPEEIMENTS ON METAMORPHISM AND 



the glass is incompletely acted upon, the centre, although still vitreous, 

 also shows very fine zones, like onyx, the whole reminding us of the 

 characteristics of certain schistose and crystalline rocks. The schis- 

 tose structure which the glass tubes acquire is evidently the effect of 

 their mode of fabrication, which has given to the mass a structure by 

 superposed layers. It is a species of heterogeneity which can be 

 made evident by the subtle action of polarized light, but which, to 

 the naked eye, is at first concealed by an apparent homogeneity. It 

 appears when water, by an unequal action, has marked out the zones 

 of different natures, and still better when the substance already modi- 

 fied in part has undergone contraction. These lamella) are, in effect, 

 more apparent in some tubes than in others. A manifest proof of a 

 fact of which we find different examples in manufacturing,* and which 

 shows how the primitive disposition of the constituent materials of a 

 rock, although they may have disappeared by ulterior action, may 

 still be present in a latent state, and reveal themselves when new in- 

 fluences bring them to light, t 



A phenomenon which almost always accompanies a schistose struc- 

 ture in crystalline rocks, is the remarkable parallelism which a part 

 of these crystalline elements presents.:]: Those which have the form 

 of spangles, whatever may be their nature — mica, chlorite, talc, 

 graphite, or specular iron — are arranged flatwise, according to the 

 planes of their lamellEe; sometimes they even present a sort of align- 

 ment, which has been called linear parallelism, as if there had been a 

 drawing out. The chlorite, micaceous, and talcose schists offer, the 

 best characterized examples of this phenomenon. 



It is even to these spangles, which were supposed to assume an 

 arrangement in parallel planes under the influence of calorific or 

 magnetic action, that, in accordance with the views of Sir John Her- 

 schel,§ the cause of the lamellar structure has often been attributed. 

 Sorby has sought to confirm this influence of spangles by an ex- 

 periment which consists in laminating a mass of paste containing 

 them. I cannot accede to this view, and I think that the alignment 

 of the spangles, instead of being the cause, is, on the contrary, only 

 the consequence of the pre-existence of these lamellte. I found my 

 opinion on four principal reasons : 1st. A lamellar structure is some- 

 times developed in nature, and I have seen it perfectly produced, in 

 the experiment previously cited, in the absence of all kinds of span- 

 gles. 2d. Crystals which are far from having the lamellar form, as 



* We know, for instance, that in shaping pieces of porcelain clay, the person who does 

 the rough work at the lathe and the moulder must take great care to produce an equal 

 pressure on all the parts of the piece which they are executing. Very small inequalities of 

 pressure, at first completely imperceptible, become evident after baking by numbers of 

 irregularities, some of which are known under the name of screwing, (vissage.) — (Brongniart, 

 Arts ciramiques, vol. i, p. 123.) 



The fabrication of Chinese image mirrors appears to be founded on a principle of the 

 same kind. 



t The experiments which I have recently made bring out the schistose structure in a 

 metal which would seem as little disposed to acquire it as brass. 



{ All the schistose minerals and rocks do not show this alignment; thus the macles are 

 not in general disposed parallel to the layers of the phyllades, of which they form a part. 



§ Manual of Geology, Lyell, 1857, vol. ii, p. 447. 



