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152 GEOGNOSY. - [Boor IT. 
form of the mineral serpentine to whatsoever cause its mode of 
formation may be assigned. 
Serpentine,’ a compact or finely granular, faintly glimmering, or 
dull rock, easily cut or scratched, having a prevailing dirty-green 
colour, sometimes variously streaked or flecked with brown, yellow, 
or red. It is a massive form of the mineral serpentine, but frequently 
contains other minerals. One of its commonest accompaniments is 
chrysotile or fibrous serpentine, which in veinings of a silky lustre 
often ramities through the rock in all directions. Other common 
enclosures are olivine, bronzite, enstatite, magnetite, and chromic iron. 
Serpentine occurs in two distinct forms; first, in beds or in- 
definitely-shaped bosses, intercalated among schistose rocks, and 
associated especially with crystalline limestones; second, in dykes or 
veins traversing other rocks. 
As to its mode of origin, there can be no doubt that in some 
cases it was originally an eruptive rock. In the Old Red Sandstone 
of Forfarshire and Kincardineshire it is found in dykes traversing the 
sandstones and conglomerates. The frequent occurrence of recogniz- 
able olivine crystals or of their still remaining contours in the midst 

Fic. 26.—Microscopic STRUCTURE OF SERPENTINE (20 Diameters). 
of the serpentine matrix affords likewise good grounds for assigning 
an eruptive origin to many serpentines which have no distinctly 
eruptive external form. The rock cannot of course haye been 
ejected as the hydrous magnesian silicate serpentine, but it may have 
been originally essentially an olivine rock, and as such may have been 
injected in the form of sheets or dykes into the overlying crust. But, 
on the other hand, the intercalation of beds of serpentine among 
schistose rocks, and particularly the frequent occurrence of serpentine 
in connection with more or less altered limestones (West of Ireland, 
Highlands of Scotland, Ayrshire), suggests another mode of origin 
in these cases. Some writers have contended that such serpentines 
' See Tschermak, Sitz, Akad. Wien, lvi. July, 1867; Bonney, Q. J. Geol. Soc. xxxiii. 
p. 884, xxxiv. p. 769; Geol, Mag. vi. p. 862; Michel-Lévy, Bull. Soc. Géol. France, vi. 
3rd ser. p. 156, wii 
