220 T. S. Hunt on the Chemical and Mineralogical 
versal alteration of areas of sedimentary rocks, embracing many 
iles. h 
hundred thousands of square miles 
well shown) are confined to certain sedimentary deposits, ap 
to definite stratigraphical horizons; above and below which, sa- 
a a ucing 
chernical changes only in those strata in which soluble alkaline 
salts are present.° 
Wh 
been broken up, and the periods during which they have Te 
mained unmetamorphosed and exposed to the action of infiltra- 
* It should be remembered that normal or regional metamorphism is in no WAY 
dependent upon the proximity of unstratified or igneous rocks, which are rarely 
present in metamorphic districts. The ophiolites, amphibolites, euphotides, d tes, 
and granites of such regions, which it has been customary to regard as ne 
trusive rocks, are in most cases indigenous, and are altered sediments. I have else- 
where shown that the great outbursts of intrusive dolerites, diorites and trachytes, 10 
deavored to explain this by the consideration that the great volume of overlying 
ed j cent ‘ede 
strata now exposed by denudation, produced a. depression of the earth's suria’ 
Jn lines of fracture whieh 
: : ond. See my paper “On some Points in Ameria 
Geology,” American Jour. Science, 2], xxxi, 414. a cae 
® See Report of the Geological ey of Canada, 1853-6, pp 479, 480; OF 
Canadian Naturalist, vii, 262. For a consideration of the relations of mimery 
cal formations, see Report on the Geology Canada os 
{now in press), p. 61; also chap. xix, on “Sedimentary and M = par : 
where most of the points touched in the present paper are di 
