GEOLOGICAL HISTORY OF SERPENTINES. 174 
ably present in the latter. This distinction is probably not absolute, since chromite is said 
to occur in the serpentine of New Rochelle, and chromiferous minerals have been found 
in the Laurentian rocks in Canada. 
$ 17. The serpentines next to be noticed occur in very different lithological associa- 
tions from the last, and in a group of rocks which has been described under the name of 
Huronian. These may be defined as in large part greenish hornblendic schistose rocks, 
passing in the one hand into massive greenstones, diorites or gabbros, and on the other hand 
into steatitic, chloritic and hydromicaceous, or so-called talcose or nacreous schists, some 
varieties of which resemble ordinary argillites, with quartzose layers, often with epidote, 
and with associated beds of ferriferous dolomite and magnesite. In this lithological 
group (already referred to in § 8) which is now known to mark a definite geological horizon, 
the serpentines are found interbedded, sometimes mingled with carbonate of lime or of 
magnesia, but seldom or never presenting varieties like the granular ophicalcite of the 
Laurentian. To this horizon belong the serpentines of eastern Canada, found in the conti- 
nuation of the Green Mountain range (the altered Quebec group of Logan), as well as those 
of Newport, Rhode Island, and apparently those of Cornwall, Anglesea and Ayrshire in 
Great Britain. The serpentines of this series are darker coloured than the last, and gener- 
ally contain small portions of chrome and nickel in combination. * 
§ 18. Serpentines are also met with in eastern North America in somewhat different 
associations from the two foregoing groups, and apparently belonging to a third geological 
horizon. The determination of the precise stratigraphical relations of the serpentines in 
question presents, however, certain difficulties arising from considerations which will be 
made apparent in the sequel. Serpentine, though not exempt from subaérial decay, resists 
this process better than hornblendic, feldspathic and calcareous rocks. Hence it happens 
that in regions where these are decomposed and disintegrated to considerable depths, asso- 
ciated masses of serpentine may be found rising out of the soil, without any evidences of 
the precise nature of the rocks which once enclosed them. Illustrations of this condition 
of things are found in the vicinity of Westchester and of Media, in Chester county, Penn- 
sylvania. The underlying rocks in this region are known to be chiefly gneisses, with horn- 
blendic and mica-schists, and include what are believed to belong to two distinct series, 
both of which are well displayed in the section seen on the Schuylkill River, below Nor- 
ristown. Here the older Laurentian gneiss, such as it appears in the South Mountain and 
the Welsh Mountain, comes up in Buck Ridge, while the newer gneiss and mica-schist series 
is seen succeeding it to the southward, at Manayunk and Chestnut Hill, at which latter 
locality it also appears on the north side of the narrow Laurentian belt. In this section, as 
it is exposed on the Schuylkill, a belt of serpentine, with steatitic and chloritic rocks, appears 
between the two series, but elsewhere it is wanting along the outcrop of the older gneiss. 
In the localities farther west in Chester county, already mentioned, at Westchester and 
Media, where the rocks adjacent to the serpentine are disintegrated, and have disappeared, 
from decay, it cannot be determined whether these serpentine-masses belong to the older 
or the newer series—which latter appears to be similar to that including the serpentiue and 
chrysolite rocks of Mitchell county, North Carolina. ($ 123). 

* For an account of these serpentines, see Geology of Canada, 1863, pp. 472, 608-612; also Contributions to 
the History of Ophiolites (1858), Amer. Jour, Sci. (2) xxv., 217-226. 
