


CRETACEOUS AND TERTIARY—PORCUPINE CREEK. 97 
_The rocks show no well marked dip, but appear to undulate slightly 
at very low angles. 
233. The occurrence of gypsum as selenite was observed, here and 
elsewhere, to be nearly always in association with plant-beds, and gener- 
ally with those holding many half-obliterated vegetable remains, and of a 
purplish shade. This association is not accidental, but brought about by 
the action of the included vegetable matter, which when still under- 
going decomposition, would facilitate the deposition of iron pyrites by 
the deoxidation of iron compounds in presence of organic sulphur com- 
pounds. The porous character of the plant-beds, at a subsequent period, 
would allow the passage of surface waters containing oxygen; leading to 
the decomposition of the pyrites, the formation of sulphuric acid, and by 
its action on the clays, of selenite or calcic sulphate. 
234. A few miles west of Pyramid Creek several hills are capped 
with heavy and hard sandstone beds, a feature quite exceptional in a 
country so gently undulating. These do not appear to be perfectly hori- 
zontal, but have a slight dip to the west. They may be equivalent to the 
capping sandstone of the Pyramid Hill, but more probably are yet higher 
in the series. 
Ten miles west of Pyramid Valley, in the upper part of the valley of 
another stream, yellowish and grey stratified sandy clays, are again seen, 
but are not perfectly exposed. 
With the exception of this exposure, the underlying rocks are 
nowhere clearly visible in the vicinity of the Line from Pyramid Valley 
to Porcupine Creek, a distance of about thirty-five miles. One very con- 
siderable stream is crossed about midway, but its valley is wide, and with 
gently sloping banks. Highlands appear to the north, and may possibly 
show sections of strata overlying those seen in the banks of the streams, 
but I was unable to reach them, the ground, especially in the vicinity of 
these highlands, being covered with snow, and the time at my disposal 
limited. 
Lignite Tertiary Rocks of Porcupine Creek. 
235. In Porcupine Creek and the tributary vallies, many partial 
sections occur. Lignite is seen in three places near the Line, and just 
above the level of the brook in each instance. The exposures seem to 
belong to the same bed, and if so, nearly a mile of its horizontal extent 
7D 
