The rocks of this series are mostly sands,
clays, and limestones--_Chalk_ itself being unknown except in
Western Arkansas. Amongst the sandy accumulations, one of the
most important is the so-called "marl" of New Jersey, which is
truly a "Greensand," and contains a large proportion of glauconite
(silicate of iron and potash). It also contains a little phosphate
of lime, and is largely worked for agricultural purposes. The
greatest thickness attained by the Cretaceous rocks of North
America is about 9000 feet, as in Wyoming, Utah, and Colorado.
According to Dana, the Cretaceous rocks of the Rocky Mountain
territories pass upwards "without interruption into a coal-bearing
formation, several thousand feet thick, on which the following
Tertiary strata lie unconformably." The lower portion of this
"Lignitic formation" appears to be Cretaceous, and contains one
or more beds of Coal; but the upper part of it perhaps belongs
to the Lower Tertiary. In America, therefore, the lowest Tertiary
strata appear to rest conformably upon the highest Cretaceous;
whereas in Europe, the succession at this point is invariably an
unconformable one. Owing, however, to the fact that the American
"Lignitic formation" is a shallow-water formation, it can hardly
be expected to yield much material whereby to bridge over the
great palaeontological gap between the White Chalk and Eocene
in the Old World.
Pages:
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395