1866.] KING iND EOWNEY " EOZOONAL EOCK." 187 



II. General Descrijytion of the Chemical, Mineral, and Structural 

 Characters of Orenville " Eozoonal " Ophite. 



In a chemical point of view this rock consists essentially, ls|_, of 

 hydro -magnesian silicates, the base being often more or less com- 

 bined with alumina or iron-protoxide ; and, 2nd, of carbonate of lime^ 

 a portion of which is sometimes replaced by carbonate of magnesia. 

 Mineralogically, it consists of serpentine, pyrosclerite, or loganite, 

 including their respective varieties, which may be referred to the 

 first group of substances* ; also calcite and dolomite, which belong to 

 the second group. These may be regarded as its essential constituent 

 minerals ; but there are occasionally present diopside, pyrallolite, 

 chondrodite, apatite, mica, pyrites, graphite, giobertite, and others. 



Polished specimens of " eozoonal " Ophite, examined by an ordi- 

 nary magnifier, are seen to consist of a more or less saccharoidal 



* Several variously named ophitic minerals are known, which, although differ- 

 ing structurally, are identical in chemical composition, (a.) Serpentine is amor- 

 phous, uneven or conchoidal in fracture, and composed, according to Scheerer's 

 analysis of pure specimens from Snarum in Norway, of Si O3 40'74, Mg O 41 "48, 

 HO 12-64, &c. 48-2. Retinalite (Thompson), although stated to contain two or 

 three per cent, more water, is considered by Sterry Hunt to be the same mineral. 

 Bowenite, which occurs as scaly aggregations imbedded in saccharoidal calcite, 

 may be regarded in the same light, (b.) Chrysotile (Kobell), found in layers 

 often very thin, with a transverse fibrous structure identical with that of ami- 

 anthus or the finest asbestus, is composed, according to Kobell' s analysis of 

 spacimens from Reichenstein, Silesia (-the original locality), of Si O^ 43'50, 

 Mg O 40-00, HO 13-80, &c. 248 : it passes into picrolite, in which the fibres are 

 coarse or acicular, adherent laterally, or separated, and not always parallel, 

 (c.) Metaxite (Breithaupt) occurs in tufts or dendritic aggregations of a dull 

 or slightly silky lustre, and consists, according to Delesse, whose specimens were 

 from Reichenstein, of Si O3 421, Mg O 41-9, HO 13-6, &c. 2-4. Some other 

 varieties are known, as thermophyllite and marmolite, having a similar chemical 

 composition. All the foregoing may be regarded as allomorphs of one and the 

 same mineral species, consisting, if we eliminate the minor ingredients, of 

 "Silica 43, Magnesia 44, and Water 13" (Delafosse), or 2Si03, 3Mg O, 2 HO. 

 There also occur a number of other hydrous silicates of magnesia related to the 

 one above formulated, agreeing with it in the percentage of HO, but differing 

 therefrom in containing Al^ O3, or Fe2 O3, or both, or Ca O, — namely, pyrosclerite, 

 bastite, loganite, and chonicrite — every one, like serpentine, having its allo- 

 morphs. Besides, there are allied species containing more or less water, as 

 deweylite, picrosmine, and others. Confining ourselves to serpentine, pyro- 

 sclerite, and loganite, these being essentially ophitic, they are respectively re- 

 presented by an earthy variety — namely, aphrodite, saponite, pseudophite, and 

 some containing an extra percentage of water. Minerals, it is well known, often 

 present themselves under various forms without undergoing any chemical change, 

 and without any relation to the different axial systems in which any one of them 

 may crystallize. Thus, aragonite occurs (1) compact and amorphous; (2) mas- 

 sive and cleavable ; (3) coralloidaJ , as flos ferri ; (4) asbestiform, as satin spar. 

 All these varieties are considered to belong to one axial system — the raonocli- 

 nate ; but being obviously of other forms than one, and as they are all chemi- 

 cally alike, we propose to call them allomorphs — from aAXos, other, and fjiop^rj, 

 form. Calcite is of the same chemical composition as aragonite ; the fact, how- 

 ever, of its belonging to a different axial system — the rhombohedral — makes it a 

 dimorph, and not, as in the above sense, an allomorph. Like aragonite, calcite 

 is also subject to allomorphism; and so are a large number of other minerals. 

 Not being aware that any distinctive term has been applied to the first class of 

 precited modifications, the want of which has often been felt in our present in- 

 vestigations, we have been induced to propose for it the one already employed. 



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