GEOLOGICAL HISTORY OF SERPENTINES. 211 
In this connection may be noted the well-known fact of the aqueous deposition of 
serpentine in veins, inthe forms of marmolite, picrolite and chrysotile, either alone or with 
calcite. Such veins, the result of a secondary process, are often found intersecting ophical- 
cites and serpentine rocks at various horizons, and are even met with in comparatively 
recent serpentine-breccias, as noticed by Gastaldi () 42.) 
VII—STRATIGRAPHICAL RELATIONS OF SERPENTINES. 
§ 126. The contradictory opinions expressed by different observers as to the geognosti- 
cal relations of serpentine-rocks in a given area—one regarding them as indigenous and 
another as exotic masses—make it evident that certain appearances are differently inter- 
preted according to the theoretical point of view of the observer. In greatly crushed and 
displaced strata the varying resistance of unlike rocks undoubtedly gives rise to accidents 
which are regarded by many as evidences of posterior intrusions. 
The serpentines and related rocks of Carrick in Ayrshire, Scotland, may be cited 
as another instance of this conflict of opinion. As described by James Geikie in 1866,* 
the serpentine and its associated greenstones are both indigenous bedded rocks interstrati- 
fied with greenish crystalline schists which he, following Murchison, called altered Lower 
‘Silurian. Geikie, however, found what he regarded as clear evidence that these strata had 
been greatly disturbed while in a softened condition. The remarkable resemblance 
between these crystalline schists of Carrrick and those associated with the serpentines of 
Cornwall, is noticed by Warrington Smyth. Bonney, in 1878, + rejected the conclusions 
of Geikie, asserting that we have in Carrick, as elsewhere, truly eruptive serpentines, 
followed by eruptive gabbros of two ages, and like Geikie, adduceed evidence in support 
of his own views. 
§ 127. In a critical notice, in 1878, of Prof. Bonney’s description of the serpentines of 
Cornwall and of Ayrshire, the present writer said: “ When it is considered that there is 
abundant evidence that the North-American serpentines are indigenous, though often, like 
deposits of gypsum and of iron-ores, in lenticular masses ; and further, that the movements 
which the ancient strata have suffered, have produced great crushings and displacements, 
it is not difficult to understand the deceptive appearance of intrusion which these rocks 
often exhibit, and which are scarcely more remarkable than the accidents presented by 
coal-seams in some disturbed and contorted areas.” + 
The alternately thickened and attenuated condition of coal-seams in such districts, 
and the forcing of the coal into rifts and openings in the enclosing sandstone-strata, is 
familiar to those who have studied the contorted measures of the Appalachian coal-field. 
The latter phenomenon especially is well displayed in one of the elaborate sections made 
since 1878 by Mr. Charles A. Ashburner, and just published by the geological survey of 
Pennsylvania, in which the so-called Mammoth-vein is shown as it occurs in the Green- 
wood basin of the Panther-Creek district.) The accidents in this great forty-foot seam of 
anthracite, here represented on a scale of one inch to 400 feet, are such as would, in a rock 
of conjectured igneous origin, be deemed strong evidence of its intrusive character. 


* Geol. Journal, xx., 527. + Ibid, xxxiv., 789. 
Ÿ Harper’s Annual Record, 1878, p. 293. 
4 Second Geol. Survey of Penn., vol. L., Southern Coal-Field ; Cross-Section Sheet ii., Section 10. 
