the grand canyon of arizona-第21部分
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are of immense interest to the geologist； as they are the first known rocks containing life to emerge from the primeval sea。 Within the last few years； they have been called the Algonkian Series； and later I shall speak of them more freely。
Prior to the deposition of these Algonkian strata； the Laurentian rocks （the granite） upon which they rest were subject to a long period of 〃planation；〃as the grinding down and leveling of rock surfaces is termed。 After this planation was complete； a subsidence occurred； the whole area became the bed of an inland sea； and upon the planed…down granite； the debris that formed the Algonkian strata was washed。
While they were being deposited； the whole region was the scene of several seismic and volcanic disturbances； for great dykes and 〃chimneys〃 of lava are found； showing clearly that； by some means or other； the strata were broken and shattered； cracked and seamed； and that through these cracks the molten lava oozedforced up from the interior of the earth。 It spread out over the Algonkian rocks in small sheets or blankets； which here and there are still to be found to…day。
Tilting of the Algonkian Strata。 Slowly this twelve thousand feet of strata emerged into the sunlight。 In the uplifting processes； the surface of the earth； where they were； became tilted； and these strata therefore 〃dipped〃 or 〃tilted〃 away from the perpendicular。 As they emerged； weathering and erosion began。 It is most probable that this process of degradation began and continued while the topmost strata were at or near sea level； so that it was a simultaneous process with the uplift。
Erosion of the Algonkian。 How many centuries this weathering and washing away process consumed no one knows。 At the close of this epoch； however； the Algonkian strata had been eroded almost away； owing to its tilted condition； so that in some places even the surface of the Archaean was exposed； and suffered the planing…down process。 Figure 1 on plate facing page 98 is a suggestion as to the possible appearance of the rocks at this time。
Even then； in those far…away； early ages of history； if one had been present to measure these strata； he would have discovered the astounding fact that； although he had measured them and found twelve thousand feet before they began to emerge from the ocean； there were but about five hundred feet of them left。 This is one of the interesting facts in geology；that an observant reader can deduce so much from so little。
The twelve thousand feet deposit。 〃But；〃 asks the layman； 〃I cannot possibly see how； if only five hundred feet of strata are left； any one could ever tell that there were once twelve thousand feet。 If eleven thousand five hundred feet are gone； how do you know they ever existed？〃
A very reasonable question and one very easily answered。 Refer to the sketch。 Let the bracket on the right show the present width of the remaining strata； viz： five hundred feet。 Now observe the tilted condition of the remnants。 To get the original height of the depositions begin with No。 1； the stratum nearest the Archaean and measure that。 Suppose it gives us five hundred feet。 No。 2 gives two hundred feet； No。 3； five hundred feet； No。 4； one hundred and seventy…five； and so on up to No。 14。 As these strata were deposited horizontally； all we have to do is to mentally replace them in their horizontal position。 Throw the tilted strata back again into their original condition； and by this method of measurement it is seen that the twelve thousand feet can be made up。 Figure 2； facing page 98。
Another interesting question here arises： 〃What became of the vast quantity of sand and silt and pebbles that formed and were carried away during such a gigantic process？ For； think of it； eleven thousand five hundred feet of strata； or rock； two miles high； almost three times as high a mass as the present distance in vertical height from El Tovar to the river！ Where has it all gone？〃
Naturally an answer to these questions is mere conjecture； as only from a study of the facts revealed underneath the present strata； can any comparative knowledge be gained of the conditions existent at that prehistoric age。 There may have been one river； or a score； or any number between； and it is probable one or more rivers carried the Algonkian debris westward and deposited it； as the Colorado River （not brought into existence until centuries later） is now doing with the debris of the existent strata。
Another Subsidence。 Now； a new era is about to dawn。 Planed and smoothed off as they are； the Algonkian and Archaean masses are to be submerged once more in the ever receptive ocean。 A period of subsidence occurs； and the whole area is soon hidden under the face of the sea。 But； all around these are masses； some day to be mountain peaks； that refuse to sink again into the sea。 Then the forces of the air assail them。 If they cannot be drowned； they shall be gnawed at； smitten； cut and worried by the air； the chemicals of the atmosphere； the storms； the rain； the hail； the frost； the snow； and thus made to feel their insignificance。 Slowly or rapidly； they yielded to this disintegrating process； and as the rocky masses broke up； they were washed by the rills and streams into the bed of the sea； where they soon rested upon the tilted ends of the Algonkian strata and exposed surfaces of the Archaean masses； waiting for them。
The Deposition of the Tonto Sandstones。 The wise men tell us that this ocean was a salt sea； and that it was quite shallow while these new sediments were being deposited。 Little by little one thousand feet of the sediments of this epoch were washed down； so that it is very likely that the tilted strata upon which they rested slowly sank lower and lower to accommodate them。 Then； for some reason or other； there was a rest for a whilea few hundreds or thousands of yearsand the masses of sediments became cemented into sandstone and shale； which we call the Cambrian formation； or the Tonto sandstone。 This is to be seen resting both upon the Archaean and Algonkian from the porches of El Tovar。 It is composed of strata of dull buff； very different from the brilliant redsalmost crimsonsof the Algonkian； and the bright reds of the strata which later were to rest above them。
Geological Terms。 What an audacious science this geology is！ How ruthlessly it wrests aside the curtain from the mystery of the past； and how glibly it deals with thousands； millions of years； tying them up into packages； as it were； and handing them out labeled 〃eras〃 and 〃periods。〃 As usual； the names made by the wise men are hard to pronounce； and seemingly hard to understand。 But a few minutes will take away the difficulty。 They divide the eras into four； viz。： 1； Proterozoic； 2； Paleozoic； 3； Mesozoic； 4； Cenozoic。 All these 〃zoics〃 have to do with life。 Proterozoic means before life； and signifies the rocks that contain no fossils indicative of life； Paleozoic signifies the most ancient forms of life； Mesozoic signifies 〃middle life〃 or those between the most ancient and the Cenozoic； or recent forms of life。 The periods are lesser divisions of the eras。 In the Proterozoic； there are two periods； viz。： the Archaean and the Algonkian。 The Paleozoic has six periods； viz。： the Cambrian； Ordovician； Silurian； Devonian； Carboniferous and Permian。 The Mesozoic era has three periods； the Triassic； Jurassic and Cretaceous； while the Cenozoic era names five periods；the Eocene； Oligocene； Miocene； Pliocene and Pleistocene。
Absence of Certain Strata。 To shorten our story； let me at once say that during the periods that the Ordovician； the Silurian and the Devonian were forming； the Grand Canyon region was either above water so that it received none of these sediments； or； if any were deposited； they were almost entirely removed by the weathering processes before described； ere the region again sank into the ocean to receive the deposits of the Carboniferous epoch。
The Carboniferous。 During this latter period； more than three thousand feet of strata were deposited。 These are the most striking in appearance of all the Canyon strata； for they reach from the Tonto shales to the rim； and consist of three principal strata （with many smaller ones in between）。 The largest is the red…wall limestone； which constitutes the base of nearly all the architectural forms found in the Canyon； and is the thickest of all the strata。 It presents the 〃tallest〃 wall of the series。 The two separate walls； one above the other； on the top of the Canyon； as seen in the arms of the amphitheatre at El Tovar； are the other two wide members of this Carboniferous period。 The lower is the cross…bedded sandstone； and the upper the cherty limestone。 There is a remarkable difference in the appearance and the material of which these Carboniferous strata are formed； and those of the East and Europe。 We generally think of coal…bedscarbon when this period is mentioned。 Here there are none。 In the East； in England； and in other parts of Europe； vast marshes existed in this period； and the rank vegetation of these marshy areas formed the coal…beds； with which the Carboniferous there abounds。 It is only by the fossils found that the periods to which the various strata belong are determi
