EFFECTS OF METAMORPHISM. 25 
changes. The nature of the intruding rock, however, has had much to do with the degree 
of metamorphism. The diabase dikes effect in the wall rocks very little change beyond 
a rather slight induration. Granite, on the other hand, has produced very pronounced 
changes. Little opportunity was found to examine the effects of the large granite masses, 
but in the case of some of the smaller dikes the results can be well studied. 
In amphibolite the secondary, uralitic hornblende is generally converted into a biotite 
which under the microscope possesses the clear, chestnut-brown color characteristic of 
mica of this origin. In a few cases a lavender amphibole, related to glaucophane, accom- 
panies this secondary biotite. It appears in long, slender prisms of amphibole outline, 
with cleavage not so perfect as is common in hornblende. A transverse parting is also 
present. The plane of the optic axes is the clinopinacoid and the maximum value of 
the angle r:e is 16°. It seems probable that this orientation of the axes of elasticity is 
somewhat variable, for most sections give a much smaller extinction angle. The char- 
acter of the double refraction is positive. Pleochroism is noticeable, as follows: a=pale 
yellow to colorless, li=very delicate lilac, r=lavender; £^\x>u. Some of the amphi- 
bolite is tinged with a border of a green, strongly pleochroic mineral, of fairly strong double 
refraction, which may be another amphibole intergrown with the lavender variety. Small 
red garnets are developed in some abundance, tourmaline and magnetite are introduced 
in certain cases, and not uncommonly small grains of sulphides, pyrite, pyrrhotite, and 
chalcopyrite, many of them intergrown, impregnate the wall rock. These changes become 
less prominent away from the contact, but the secondary biotite has been found more 
than 10 feet from the granite. 
The effect on rocks other than amphibolite has not been carefully studied. The prin- 
cipal result seems to be the development of the brown biotite. 
Pegmatite as a rule acts in a similar way to granite, except that its action is in most 
cases much less pronounced. In a few instances, however, tourmalinization of the imme- 
diately adjoining wall rock has been exceedingly intense. Garnets and sulphides are sel- 
dom seen at the contact of pegmatite dikes. 
The metasomatic replacement effected by quartz-vein solutions in their wall rocks is 
in Ihese southern deposits strikingly like that produced by contact metamorphism. This 
feature is dwelt on at greater length under the heading "Gold" (pp. 63-64), but it may be 
said at this point that tourmaline, garnet, magnetite, and sulphides characterize the wall 
rocks of many of the quartz veins and the resulting products are in some cases almost 
indistinguishable from those caused by granite or aplite dikes. Porous rocks, like the 
acidic volcanic tuffs, have in many places been subjected to intense silicification. 
All these results of contact metamorphism bear evidence that they were produced at 
great depths and have been exposed only by profound erosion. 
HYDROMETAMORPHISM. 
Throughout the southern Piedmont region the effects of weathering are very prominent. 
In many places marked decomposition of the rocks has extended over 100 feet below the 
surface. It is believed that the physiographic history of the region has allowed this weath- 
ering to proceed to so great a depth. With the slow rate of erosion as base-level was 
attained and the help of those conditions which are supposed to exist at such a stage, 
decomposition of the rocks exceeded erosion of the products of decomposition and in con- 
sequence extended to very unusual depths. This decomposition seems to have been, except 
perhaps within a few feet of the surface, much more a hydration than an oxidation — a 
fact which probably indicates that there was sluggish circulation of the waters that caused 
this change. So far as they have been studied, the principal products of this hydration 
are kaolin and chlorite. Magnetite is oxidized to limonite near the surface and colors 
red or brown all the rocks which contain it. The amphibolites thus produce a very red 
soil, while much of the magnetite-bearing itacolumite is stained red or pink. The quartz- 
ites become friable, partly from solution of the cementing silica and partly from decompo- 
sition of the mica which helps bind the grains together. The limestones are dissolved, 
