i 
pad OO 
“Again 
_ meteoric influences, a kind of lustrous q' 
Le. D. Irving—st. Peters and Potsdam Sandstones. 403 
investigation, evidence of the nature of the metamorphism 
mvolved in their formation,’ The frequent gradation of the 
Most compact, non-granular, often vitreous quartzites into 
plainly arenaceous and even pebbly, water-deposited rocks, 
and the small number of minerals and consequent  proba- 
ble simplicity of any chemical reactions involved, encour- 
aged me to hope that I might find some clue to the solu- 
formation is more indurated in its lower portions. The amount 
of induration varies greatly, in some cases being a barely per- 
ceptible hardening, and again ranging through various degrees 
to an extreme in which the rock becomes a compact almost 
Vitreous quartzite, the thin slabs ringing like steel’ when struck 
ith a hammer. Now it is certain that in such an unaltered 
ordinary orthodox regional metamorphism through which the 
crystalline schists are generally believed to have passed. More- 
Over, I had often observed a peculiar hardening and vitrifica- 
tion of both this sandstone and that of the St. Peters, on ex- 
posed surfaces, which is plainly a result of weathering and 
therefore of a necessity wholly unlike such a thing as a general 
Te-crystallization. This peculiar effect of weathering, which I 
av i 
* In Geikie’s Text Book of Geology, pp. 158, 333, the exposed blocks of Eocene 
‘Sandstone which are known as “grey wethers” in Wiltshire, and which occur 
in in the region of the Ardennes in France, are spoken of as becoming, under 
uartzite. 
. 
