!r-;--?3- PivANETEslMAL THOR\'. This theory of earth origin gives promise of displacing 

 the two preceding, but is itself still in its formative stage. It assumes that our present solar 

 system was derived from an original gaseous mass (a sun and star} which was moving through 

 space and revolving upon an axis. By the dose approach of another great body this gaseous 

 mass was partially disrupted by its own elastic force and the gravitation of the second body. 

 The arms of matter shot out, containing variable masses (knots) and much finely divided mat- 

 ter (plandesimals), were twisted into a spiral similar to the spiral nebulae now seen in the 

 heavens. The disrupted material from the central sun, constituting not more than one to two 

 percent of its original mass, was given a motion about the sun in elliptical orbits. It soon 

 passed into the liquid or solid state and some of it remained hmiinous,either because of heat or 

 from some other possible cause. The minute plauetesimals would slowly collect about the 

 knots as nucleii, more from their overtaking one another than from gravitation. The energy 

 of the collisions would be so slight and their accumulation so slow that no significant amount 

 of heat would be added to the newly forming planet. In some such way is the earth supposed 

 to have originated. As the spheroidal mass slowly' grew in size the condensation of its inter- 

 nal portions by the tremendous weight of the outer layers would liberate heat and the interior 

 of the earth became warmer and warmer. This heat and pressure forced to the outside the 

 gases of which the atmosphere is composed as well as that which makes up the seas. The 

 moon is supposed to have been formed from a neighboring knot of matter, but to have grown 

 less rapidly than our earth and to be still too small to hold atmospheric gases by its gravita- 

 tion. Hence it is practically without water and an atmosphere. 



B. History of the Earth. The geological history of the earth may be assumed to have 

 begun with the formation of a solid crust, providing the molten condition is accepted. Pre- 

 vious to this time it was simply a small star instead of a planet. According to the planetesi- 

 mal hypothesis the earth may be assumed to have begun its geological history with the acquisi- 

 tion of an atmosphere. This has been computed to be when it had attained a diameter of 

 about 4200 miles. The earth, from either of these stages, has attained its present features 

 and forms of life by a process of slow, progressive change. The following stages may be 

 recoguixed. 



1. Azoic ERA. This is the life/ess stage of the earth's history, made so either because of 

 the excessive heat upon one hand or of the extreme cold and scanty atmosphere. According 

 to the nebular theory the crust must cool from over 2500 F to about 500", when the water of 

 the oceans might begin to condense under the great atmospheric pressure. According to 

 the rival theory the earth continued to grow in size from the accession of planetesimals and to 

 acquire heat from the compression of the interior. The rocks are mainly crystalline in struc- 

 ture and have been folded and crumpled by pressure. 



2. Eozoic ERA. The name means the dawn of lije, since we have indirect evidence 

 that low forms of both plant and animal life had been introduced upon the earth. This evi- 

 dence is supplied by beds of limestone, iron ores, and carboniferous shales produced to-day 

 mainly through the activity of organisms. Direct evidence of life has been found in a few 

 places, consisting of the remains (fossils} of simple representatives of groups of backboneless 

 animals (invertebrates), which lived in the sea. We have no positive clue as to the source of 

 this life. It may have been transferred horn, some other planet or it may have been created hep/. 

 Some have held that the original life was created " spontaneously" ; others that it was created 

 by DEITY. If created by the latter, two methods may be conceived; viz., direct creation, by 

 divine command, of repre;-entatives of the various groups of plants and animals; or such crea- 

 tion of only the simplest forms and their slow modification into higher groups, as the monoto- 

 nous ages crept by. 



3. PALAEOZOIC ERA. ''Ancient life." During this era conditions were much more 

 favorable for both marine and land life and for its preservation in the sediments that were to 

 be transformed into the firm outer layers of the earth's crust. Life made a great advance from 

 the sluggish, segmented creatures that crept over the slime of the sea-bottom to the vertebrated 



nimals that were able to leave the water and enjoy the forested and fern-eovered banks. Four 

 ain divisions(periods)of the era may be recognized, each characterized by successively higher 

 orms of life. The life forms of each period are related to those of the preceding, as well as 

 he following period, but are sufficiently different so that the expert can distinguish them 



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