GEOLOGICAL HISTORY OF SERPENTINES. 205 
hundred feet in thickness, which are interstratified in the Laurentian gneissic series of 
North America, and have been traced in continuous lines of outcrop for hundreds of miles, 
have resulted from such an entire transformation of corresponding portions of the granitic, 
eneissic and pyroxenic rocks of the series. * These very ingenious writers further 
imagine that serpentine also, to which they assign, in accordance with the received 
views of this school, an origin by metasomatosis (or, as they call it, methylosis), from 
dolerite, melaphyre, diorite, euphotide, and other supposed plutonic rocks—is itself subject 
to a similar change into limestone. The existence of ophicalcites, the presence of masses 
of serpentine, and of such serpentinic structures as £ozoon Canadense, in limestone, are but 
so many eVidences to them of a still uncompleted conversion of serpentine into limestone. 
§ 110. Opposed to this view of the genesis of serpentines and limestones by change of 
substance, from plutonic rocks, is that which may be described as a general metasomatic 
theory adapted to neptunism, and which, recognizing the aqueous and sedimentary origin 
of limestone, would derive from it, by alteration, not only serpentine, but the various other 
silicated rocks mentioned above. Illustrations of this are seen in the supposed conversion 
of limestone into dolomite, and of this last into serpentine, both of which views have 
found many advocates. The probable change of limestone into granite and into gneiss, 
was suggested by Bischof, and Pumpelly subsequently, in 1873, proposed to explain the 
genesis of the bedded petrosilex-porphyries or halleflintas of Missouri by the transmuta- 
tion of a stratified limestone, of which portions are found interlaminated with the 
petrosilex. + He, at the same time, suggested a similar origin for the hematitic iron-ore 
which accompanies these porphyries. 
§ 111. With this second hypothesis of the origin of serpentines may be mentioned 
another, not, however, involving metasomatosis, which has sometimes been discussed, and 
which was suggested by the present writer in 1857, from the results of certain experiments 
on the artificial formation of silicates of lime and magnesia by the reaction between car- 
bonates of these bases and free silica in presence of heated solutions of alkaline carbonates. 
Such a reaction is not without its significance, and, as I have elsewhere shown, has doubt- 
less played a part in the local development of protoxyd-silicates in sediments in the 
vicinity of igneous rocks, and of thermal alkaline waters ; but asan explanation of the gene- 
sis of great masses of comparatively pure silicates, such as olivine, serpentine and steatite, it 
is obviously inadequate, and was abandoned by the writer in 1860 for the view maintained 
below. f Even if we could suppose the presence of sedimentary beds containing the 
requisite elements in proper proportions, it can be shown that the reactions required for 
the production of silicates were inoperative in the very regions where serpentine and 
steatite are found, since side by side with beds of these are to be met with in the Huronian 
series, in many places, beds of dolomite and of magnesite intimately mixed with quartz, 
sufficient in amount, if combined, to convert the accompanying carbonates into corres- 
ponding silicates. 
§ 112 There remain then to explain the origin of serpentine, besides the three hypo- 

* See for a discussion of the views of this school the author’s Chem. and Geol. Essays, pp. 324-325 ; also, An 
Old Chapter of the Geological Record, by King and Rowney, 1881, chapters vii. and xii. 
+ Geological Survey of Missouri, Iron ores, etc., pp. 25-27; also the author on Azoic Rocks, pp. 194, 
Ÿ Chemical and Geological Essays, pp. 25, 297, 300. 
