AGATE. 



AGATE. 



country, it would be extremely dangerous to eat ; and, on the other 

 hand, even the dangerous A. muscamtt is a species of food in 

 Kamtchatka. 



The following characters will serve to distinguish such Agarics as 

 are poisonous or suspicious : 



1. Such as have a cap very thin in proportion to the gills. 



2. Such as have the stalk growing from one side of the cap. 



3. Those in which the gills are all of equal length. 



4. Such as have a milky juice. 



5. Such as deliquesce ; that is, run speedily into a dark watery liquid. 



6. And lastly, every one that has the collar that surrounds the 

 stalk filamentous, or resembling a spider's web. 



As to the rest, the eatable kinds that can be safely employed in 

 Great Britain are the following : 



A. campestris, the Common Mushroom (Jiff. 1), the species that is 

 so commonly raised artificially for food. This is readily known in 

 any state by its fragrant odour, by which alone it may be always 

 recognised, and the absence of which is extremely suspicious. When 

 in a very young state it resembles little snow-white balls, which are 

 called Buttons ; afterwards it acquires a stalk, separates its cap, and 

 becomes shortly conical, with liver-coloured gills, and a white thick 

 fleshy cap, marked with a few particles of gray. At a more advanced 

 age the cap is concave, the colour gray, and the gills black ; in this 

 state it is called a Flap. [MUSHROOM.] 



A. Georyii is like the latter, but its gills are always very pale, and 

 its flavour inferior. It is said occasionally to weigh as much as 14 Ibs. 



A. pratensis, or oreades, the Fairy-Ring Mushroom, is so well known 

 by its popular designation as to require no description. Well may it 

 have gained that name ; for, in former times, there would, doubtless, 

 be great difficulty in imagining how such productions could spring up 

 in a few hours in the regular rings they appear in, without the aid of 

 some supernatural agency. The use to which this species is usually 

 applied i that of being powdered and mixed with rich sauces, after 

 having been previously strung upon a line, and dried in the shade. 

 Dr. Badham, in his work on 'The Esculent Funguses of Britain,' 

 shews that a large number of other species may be eaten with 

 impunity. Great caution is however necessary, and no person 

 should venture on the eating of strange species unless practically 

 acquainted with their distinctions. Dr. Badham's work contains 

 drawings of the species which will greatly assist those who may be 

 desirous of distinguishing the edible kinds. 



Poisonous Aijin a i. 



A'GATE, an ornamental stone used in jewellery, and for some 

 purposes in the arts: it is sometimes called Scotch Pebble. The 

 name is derived from the Greek &x<W> a stone described by 

 Th(i|.ln M.-tii-. md which, he says, came from the river Achates, in 

 Sicily, now the Drillo, in the Val di Noto. It is one of the numerous 

 ications of form under which silica presents itself, almost in a 

 otate of purity, constituting in the agate 98 per cent, of the mineral. 

 The silicious particles are not so arranged as to produce the tran- 

 uparency of rock crystal, but a translucent, sometimes almost opaque 

 substance, with a resinous or waxy fracture; and a variety of shades 

 of colour are produced by a minute quantity of iron. The same 

 rtone sometimes contains parts of different degrees of translucency, 

 and of various shades of colour; and the endless combinations of 

 these produce the beautiful and singular internal forms, for which, 



together with the high polish they are capable of receiving, agates are 

 prized as ornamental stones. Although occasionally found in other 

 rocks, they are most usually met with in that variety of the trap 

 rocks called Amygdaloid or Mandelstein, forming detached rounded 

 nodules, not cemented to the base or mass of the rock, but easily 

 separable from it, and having generally a thin layer of green earth 

 interposed, and a rough irregular exterior, as if moulded on the 

 asperities of the sides of a pre-existing cavity. The silicious particles 

 have often, but far from constantly, arranged themselves in thin layers 

 parallel to the external surface of the nodule ; sometimes the nodule 

 is not solid, but a hollow space is left in it, studded with crystals of 

 quartz ; and not unfrequently crystals of carbonate of lime and other 

 minerals, totally distinct in composition from that of the agate, are 

 superimposed on the quartz crystals. 



The theory of the formation of agates is a problem of great difficulty, 

 and we must be much further advanced than we are, in our knowledge 

 of the chemical processes of nature in the mineral kingdom, before 

 we can expect to throw any light on this very obscure subject. The 

 great supply of agates is from a class of rocks to which all geologists 

 now assign an igneous origin, analogous to that of lava in existing 

 volcanoes. The theory divides itself into two parts ; first, the forma- 

 tion of the cavities in which the agates are found ; and, secondly, the 

 filling of these cavities. With regard to the first, we have many 

 analogies from modern lavas, and from processes of art, to guide vis to 

 a pretty satisfactory conclusion. Gases are evolved in great quantities 

 by volcanoes, and if produced at the same instant with a flow of lava, 

 they would rise in bubbles in the melted mass ; but in proportion as 

 that became more viscid they would rise with greater difficulty to the 

 surface, and when it consolidated would form cavities, the shape of 

 which would be determined by the nature of the pressure of the 

 surrounding viscid lava. To account for the filling up of the cavities 

 three theories have been proposed : one supposes the silicious matter 

 to have been introduced in aqueous solution from without, and to 

 have been gradually deposited in the cavities ; another, that, in 

 obedience to some peculiar laws of attraction, it has separated from 

 the rest of the rock, and insinuated itself into the hollows left by the 

 gases ; and a third, that these hollows were filled by the sublimation 

 of the silica and other materials from the rest of the mass by the 

 action of heat. Each hypothesis is supported by particular cases, 

 which it satisfactorily explains, but there are probably as many against 

 as in favour of each ; all of them imply conditions of chemical action 

 different from anything of which we have had experience. We fre- 

 quently find, it is true, masses of silicious petrified wood in which 

 hollows of the tree have been filled with agate, not to be distinguished 

 from many nodules found in the trap rocks ; and that the matter of 

 the agate must have been introduced into the wood by aqueous 

 infiltrations there can be no doubt : but, in this case, the whole 

 substance of the sustaining mass, the wood, is penetrated by silicious 

 matter ; and the difficulty of the theory of infiltration, in the case of 

 the trap rocks, consists in the absence of any trace in the rock of the 

 channel by which the solution of silicious matter could have arrived 

 at the cavity. The following section of an agate is a good example 

 of the filling up 

 of a cavity by 

 infiltration, for 

 it is evident 

 that the sili- 

 cious matter, 

 in whatever 

 way it may 

 have arrived, 

 was introduced 

 at the point a, 

 and that there 

 was a gradual 

 deposition of it. 

 Such examples 

 would be more 

 frequently met 

 with, if there 

 was anything Agate, 



in the external 

 coat to tell us in what direction to slit the stone : this same specimen 



accidental cut in the right directic_. 



the products of volcanoes may lead to some satisfactory concli 

 for although agates have not been found in lavas, cavities in them are 

 often partially or entirely filled with minerals distinct from any in the 

 rest of the rock. , . 



Agates are often found as loose pebbles in the beds of rivers or in 

 gravtl.but in these cases they have been derived from the disinte- 

 gration of Amygdaloids, the base of which is very often subject to 

 decomposition when exposed to air and moisture and then the smcious 

 nodules fall out. They vary in size from that of millet seed to a foot 

 in diameter ; but one, two, and three inches in diameter are the most 



C Thetones distinguished by mineralogists and lapidaries by the 



