170 DR. THOMAS STERRY HUNT ON THE 
chemical elements have been segregated and combined during successive ages in the forms 
in which we now find them in the earth’s crust: in other words how, from a once homo- 
geneous mass have been separated quartz, corundum, bauxite, carbonates of calcium and 
magnesium, as well as carbonates, oxyds and sulphids of manganese, iron, zinc, copper and 
other metals. Not less important is the problem of the genesis of the corresponding pro- 
toxyd-silicates, and especially of those of calcium, magnesium and iron, which 
form, often with little or no admixture, considerable masses in the earth’s crust. Of these, 
it is unnecessary to say the magnesian rocks under consideration constitute an important 
part, and all analogies lead to the conclusion that their constituent elements have been 
brought together by aqueous processes, such as we have already indicated. 
II1—SERPENTINES IN NORTH AMERICA. 
§ 16. It is evident that if we once come to regard serpentine as a rock formed from 
aqueous sediments of chemical origin, there is no reason, 4 priori, why it may not be found, 
like limestone, dolomite or gypsum, intercalated in stratified deposits at different geolo- 
gical horizons, and with different lithological associations. Several such horizons of ser- 
pentine have been observed in North America, which will be noticed in ascending order. 
Included in the ancient gneissic series to which the name of Laurentian has been 
given, serpentine is frequently met with associated alike with beds of crystalline lime- 
stone and with dolomite. In these beds, the serpentine is often disseminated in grains or 
small irregular masses, giving rise to varieties of so-called ophicalcite. These imbedded 
masses of serpentine are sometimes concretionary in aspect, and may have a nucleus of 
white granular pyroxene. They often recall in their arrangement, imbedded chert or flint, 
and, like this substance, sometimes attain large dimensions. These serpentines occasionally 
include the calcareous skeletons of Eozoon Canadense, the silicate replacing the soft parts 
of the organism, as described by Dawson and Carpenter. Occasionally, the serpentines of 
this horizon form beds of considerable size, either pure or mingled only with small por- 
tions of calcite or dolomite. Of these, many instances are seen with the limestones of the 
Laurentian in Canada, and a remarkable example occurs at New Rochelle, on Long Island 
Sound, near New York city, where massive bedded serpentine, highly inclined and inter- 
stratified with crystalline limestone, often itself mingled with serpentine, occupies a 
breadth of about 400 feet across the strike, the whole being conformably interstra- 
tified with massive gneisses and black hornblendic rocks with red garnet. * The gen- 
eral characters of the serpentines found with the Laurentian limestones have been else- 
where described by the present writer. Their lower specific gravity, and generally paler 
colors, together with a larger proportion of combined water Ÿ serve, in some cases at least, 
to distinguish the serpentines of this horizon from those to be mentioned as occurring in 
the Huronian series. To this may be added, a smaller amount of combined iron-oxyd, 
and, in most cases, the absence of compounds of nickel and chrome, which are almost invari- 

* For an account of this locality see Mather, Geol. First District of New York (1842), p. 462; also J. D. Dana, 
Amer. Jour. Sci. (3) xx., 30-32. 
+ For descriptions and analyses, by the author, of Laurentian serpentines, see Geol. Canada 1863, pp. 471, 591; 
also Contributions to the History of Ophiolites (1858), Amer. Jour. Sci. (2) xxvi., pp. 234-236, 239. 

