the grand canyon of arizona-第21节

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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