The Americana; a universal reference library, comprising the arts and sciences, literature, history, biography, geography, commerce, etc., of the world . Fig. 7.— Geography of North America during theLower Cretaceous. Preceding this stage, namely,during the early Mesozoic. land areas had beenmore extensive than at any time oince map represents the tirst stage of the Creta-ceous submergence which increased to its maxi-mum, see ne.\t mao. GEOLOGY [77] Probable Unds mckrinos&tcrsL ivki\owr\. tralia and South Africa late in Carboniferoustime, wlicre but a short time previo
The Americana; a universal reference library, comprising the arts and sciences, literature, history, biography, geography, commerce, etc., of the world . Fig. 7.— Geography of North America during theLower Cretaceous. Preceding this stage, namely,during the early Mesozoic. land areas had beenmore extensive than at any time oince map represents the tirst stage of the Creta-ceous submergence which increased to its maxi-mum, see ne.\t mao. GEOLOGY [77] Probable Unds mckrinos&tcrsL ivki\owr\. tralia and South Africa late in Carboniferoustime, wlicre but a short time previously tropicalconditions had prevailed, as they do now. Vari-ous hypotheses have been framed to account forthese special conditions, some of them depend-ing upon changes of astronomical relations ofthe earth to the sun, others upon altitude oflands above sea, and one upon the constitutionof the atmosphere. The last named appears atpresent to afford the most reasonable basis ofexplanation, though the hypothesis is by nomeans established as true in all its phases. Itrests upon the physical and chemical propertiesof carbonic acid (CO:), and upon the fact thatthis substance is present in our atmosphere invery small quantities. Physical researches haveshown that even the small amount present hasa potent influence in absorbing the heat of thesun and raising the temperature of the atmo-sphere. That is to say, it thickens the atmo-spheric blanket about the earth. A slight in- FiG. 8.— Geography of North .\merica
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