14 T. STERRY HUNT ON THE GENETIC 
is found in a recent note by Prof. H. Carvill Lewis on the crystalline schists of eastern 
Pennsylvania. A belt of these which crosses the Schuylkill near Philadelphia, long ago 
described by H. D. Rogers, and since by the present writer,' includes a band of granitoid 
gneiss succeeded by micaceous gneisses and micaceous schists, often garnetiferous, compris- 
ing a layer of serpentine with steatite and dioritic rocks, the whole representing both the 
older and the younger gneissic series so well known in eastern North America as Lauren- 
tian and Montalban. The rocks in this belt, notwithstanding their stratiform character, are, 
in the opinion of Lewis, “of purely eruptive origin, consisting of syenites, acid gabbros, 
trap-granulites, and other igneous rocks, often highly metamorphosed. It is the outer peri- 
pheral portions of this zone to which attention is here directed. While the rocks are massive 
on the centre, this outer portion has been enormously compressed, folded and faulted, with 
the result of producing a tough banded porphyritic fluxion-gneiss.” Lewis supposes “a 
recrystallization of the old material under the influence of pressure-fluxion,” by which he 
conceives the feldspar to have been recrystallized. “In similar manner the biotite has 
been made out of the old hornblende, garnets have been developed, and the quartz has 
been granulated and optically distorted by the pressure.” In another example mentioned 
by him, a belt of sphene-bearing amphibolite schist, described as included unconformably 
in the mica-schists of Philadelphia, is supposed by Lewis to be “a highly metamorphosed 
intrusive dyke of Lower Silurian age. The original augite or diallage has been completely 
converted into fibrous hornblende, and the influence of pressure is shown in the perfectly 
laminated character of the schist, in the close foldings produced, and in the minute struec- 
ture of the rock.” “The chemical changes and interchanges of elements which might 
result from a loosening of molecular combinations under extreme pressure,” and their sub- 
sequent rearrangement to form new compounds, suggest to Lewis great possibilities in the 
so-called “mechanical metamorphism,” now advocated by some to replace the discredited 
dogma of chemical metamorphism, which has hitherto played such an important part 
among a school of geologists.’ € 
§ 19. Thus, while the ancient Wernerians maintained the direct deposition of granite 
from aqueous solution in a chaotic ocean, the plutonists, from Poulett Scrope in 1825 to 
Darwin, Naumann, Lehmann, and Lewis, assert the igneous origin not only of granites, 
but of gneisses and micaceous and amphibolic schists, and the followers of the Huttonian 
or metamorphic school hold an untenable and an illogical position between the two,— 
deriving the materials of both these rocks from a primary granitic mass, whose origin is 
unaccounted for, and whose supposed transformations chemistry cannot explain. 
§ 29. It remains to notice, in connection with the neptunian, the plutonic, and the 
metamorphic hypotheses regarding the sources and the geognostic relations of the crystal- 
line rocks, a view that has been proposed to explain the attitude of certain apparently exotic 
masses: which is that their present position is due neither to deposition from solution, 
nor to intrusion in a fluid or plastic condition, but to local movements which have per- 
mitted portions of rigid rock to displace and even penetrate softer and more yielding 
materials in their vicinity. Examples of this are described by Stapff as seen in the 
St. Gothard tunnel in the Alps, where great masses of serpentine have been caused to 

! See Hunt, Azoic Rocks, pp. 10-15 and 200; also Trans. Roy. Soc. Can., Vol. i. Sec. iv. p. 171. 
? H. C. Lewis, Proc. British Association, in Nature, Oct. 8, 1885, p. 560. 
