32 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull 136. 
The original grains are conspicuously outlined by a rim of iron oxide 
and surrounded by interlocking areas which are optically continuous 
with the inclosed grain. Rarely the grain itself is composed of two 
differently oriented areas, in which case the enlargement is also com- 
posed of two different areas oriented with the two portions of the grain. 
These same specimens show feldspar grains, usually with the " grid- 
iron" structure of microcline. Occasionally a hornblende fragment 
is also present. The rock is evidently derived from granitic debris 
and might more accurately be called arkose. The fresh character of 
the fragments is in general noteworthy and suggests a source not far 
distant. 
The best examples of the vitreous quartzite, the extreme phase of 
metamorphism in the sediments, are to be found in the tunnel at and 
in the neighborhood of the contact with the greenstone, and on the 
unfinished Tapeworm Railroad southwest of the Old Maria Furnace at 
the similar contact of the sandstone and felsite. At both of these 
localities the rock shows macroscopically and microscopically the effect 
of dynamic action. Of two specimens from the first locality, which 
has already been described in sufficient detail on page 27, one is a pure 
quartzite with prominent cleavage, on the surface of which iron rust is 
deposited. The thin section shows a quartzite from which every ves- 
tige of the original waterworn grains has been obliterated. The rock 
consists of interlocking areas of quartz and some interstitial material. 
This is a tine aggregate of quartz grains, and may be a remnant of the 
granulation which must have occurred as a result of the movement of 
the grains against one another. 
A similar phenomenon in the sandstone of Sugar Loaf Mountain has 
been figured by Dr. Keyes. 1 
Another specimen from the same locality, but nearer the greenstone, 
shows a rock which, because of the shearing and chloritization that it 
has undergone, very closely resembles a chlorite schist. Its clastic 
character, however, becomes quite evident under microscopic examina- 
tion. This shows it to be a siliceous rock, greatly sheared and altered 
in character by the formation of sericite and chlorite. It is the only 
section of quartzite in which chlorite has been observed. Its close 
proximity to the greenstone, in which that mineral is so abundantly 
developed, suggests a probable source of the magnesia, iron, and alu- 
mina. Tourmaline, zircon, magnetite, and feldspar are also present in 
this rock. 
The quartzite at the felsite contact is similar, both in the hand speci- 
men and in the thin section, to the first of the two specimens just de- 
scribed. Ko trace of the original grain remains. Undulatory extinction 
is pronounced and recrystallization complete. Specimens obtained close 
to the acid igneous rock differ only in the presence of a large amount of 
1 C. P. Keyes, A geological section across the Piedmont Plateau in Maryland : Bull. Geol. Soc. America^ 
Vol. II, 1891, p. 321. 
