20 DE. THOMAS ST EERY HUNT ON THE 



in a lieated condition. The applications of the doctrine of metasomatosis seem to be limited 

 only by the imagination of its disciples. 



§ 39. We now come to examine what we have called the second phase of the doctrine 

 of metasomatism, which starts, not from silicated and aluminous rocks, but from lime- 

 stones, and from these proceeds to silicated rocks. The resources of the chemist were 

 severely taxed, when it was required by the metasomatist to change a sandstone or an 

 argillite into a gneiss, a hornblende schist, or a serpentine ; but with a comparatively soluble 

 rock, like limestone, the change was less difficult to conceive. Accordingly, we find von 

 Buch, Haidinger and others teaching the conversion of limestone into dolomite, and Grustaf 

 Rose and Dana, the further change of dolomite into serpentine ; while Volger, and after 

 him Bischof, maintained the transformation of limestone into gneiss and granite. The 

 argument for this change, as stated by the latter, is instructive, as showing the ordinary 

 mode of reasoning adopted by this school. The occurrence of feldspar in the form of cal- 

 cite, according to him, "proves the possibility of carbonate of lime being replaced by a 

 feldspathic substance." He elsewhere argues that since both quartz and feldspar may 

 replace calcite, " if both changes take place together, the chief constituents of gneiss would 

 be substituted for the limestone removed." " Volger also describes instances of the asso- 

 ciation of adularia and pericline with calcite, at St. Gothard, which show that feldspar, 

 quartz and mica may be substituted for the carbonate of lime in calcite. Consequently, 

 it may be inferred that granite or gneiss may be produced from limestone in the same 

 manner." ^* 



§ 40. Akin to this view of Volger is that suggested by Pumpelly with regard to the 

 halleflinta or bedded petrosilex-porphyry of Missouri (composed chiefly of quartz and 

 orthoclase) — that this rock, as well as its imbedded magnetic and specular iron and man- 

 ganese ores, may have been derived by a metasomatic process from a limestone, parts of 

 which were replaced by the oxyds of iron and manganese, " while the porphyry, now sur- 

 rounding the ores, may be due to a previous, contemporaneous, or subsequent replacement 

 of the lime-carbonate by silica and silicates." Portions of this petrosilex are, in fact, inti- 

 mately mingled with calcite, and thin layers of crystalline limestone are also found inter- 

 stratified with the petrosilex, which, in these associations, retains its normal composition 

 of a mixture of orthoclase and quartz.*'* 



The hypothesis of metasomatism as applied to silicated rocks, endeavors to account for 

 the generation of different and unlike masses in a single crystalline terrane or series, and 

 also for certain phenomena in the transformation of detrital rocks. As applied to lime- 

 stones, however, by Eose, Volger, Bischof and Pumpelly, it seeks to explain the transforma- 

 tion of a single wide-spread rock into granite, gneiss, serpentine, petrosilex, and crystalline 

 iron-ores. These transformations once established, we should have an intelligible hypothesis 

 to accoiTut for the origin of the principal crystalline rocks. 



§ 41. We have in the preceding historical sketch endeavoured to shew that the 

 existing hypotheses regarding the origin of the stratiform crystalline rocks may be classed 

 under six heads, which are as follows : — 



" Bischof; Chemical and Pliysical Geology, 1859, vol. III., pp. 431, 432. 



" Geological Survey of Missouri, 1873 ; Iron Ores, etc., pp. 25-27. Also Hunt, Azoic Rocks, Rep. E., Second 

 Geological Survey of Penn., p. 194. 



