United States or Qatar ? Vote for the TOP Country of the Week !
Below us are the limestones and hard sandstones which we found in Cataract Canyon. This bodes toil and danger. Besides the texture of the rocks, there is another condition which affects the character of the channel, as we have found by experience. Where the strata are horizontal the river is often quiet, and, even though it may be very swift in places, no great obstacles are found.
The older tertiary strata, forming the higher isolated hills and extensive tracts of country, vary, as I have said, extraordinarily in composition: within the distance of a few miles, I sometimes passed over crystalline limestone with agate, calcareous tuffs, and marly rocks, all passing into each other, red and pale mud with concretions of tosca-rock, quite like the Pampean formation, calcareous conglomerates and sandstones, bright red sandstones passing either into red conglomerate, or into white sandstone, hard siliceous sandstones, jaspery and chalcedonic rocks, and numerous other subordinate varieties.
The waste swept in by streams contains much feldspar and other minerals softer and more soluble than quartz, and where the waves have little opportunity to wear and winnow it, it comes to rest in beds of sandstone in which grains of feldspar and other soft minerals are abundant. Such feldspathic sandstones are known as ARKOSE.
The weaker strata of the long, regular folds were eroded into longitudinal valleys, while the hard Paleozoic sandstones, such as the Medina and the Pocono, were left in relief as bold mountain walls whose even crests rise to the common level of the ancient plain.
At the base of the formation beneath the rock- salt occur the Lower Sandstones and Marl, called provincially in Cheshire "water-stones," which are largely quarried for building. They are often ripple- marked, and are impressed with numerous footprints of reptiles. The basement beds of the Keuper rest with a slight unconformability upon an eroded surface of the "Bunter" next to be described.
Then again we meet with beds of pure and impure coal, alternating with shales and sandstones, and underneath the whole, perhaps, are calcareous strata, or beds of limestone, filled with corals and marine shells, each bed distinguishable from another by certain fossils, or by the abundance of particular species of shells or zoophytes.
Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait.
We have to explain the existence of several layers of coal which have been formed over one another on the same spot at successive periods, divided by other periods when shale and sandstones only have been formed. A careful estimate of the Lancashire coal-field has been made by Professor Hull for the Geological Survey.
Binney, Sir R. Murchison, Mr. Harkness, and others, that it is in the region where the limestone is most largely developed, as, for example, in the county of Durham, that the associated red sandstones or sedimentary rocks are thinnest, whereas in the country where the latter are thickest the calcareous member is reduced to thirty, or even sometimes to ten feet. It is clear, therefore, says Mr.
The decomposition of the spores has made the shales highly bituminous, and the oil and gas have accumulated in the reservoirs of overlying porous sandstones. INVERTEBRATES. We must pass over the ever-changing groups of the invertebrates with the briefest notice. Chain corals became extinct at the close of the Silurian, but other corals were extremely common in the Devonian seas.
Word Of The Day