THE EARTH BEFORE THE CAMBRIAN 387 



probably entered into combination with the rocks to an important 

 degree, while nitrogen, an inactive gas, has accumulated until it now 

 constitutes about 79 per cent, of the atmosphere. The percentage 

 of carbon dioxide in the atmosphere is about .03 of one per cent. 



The atmosphere as now composed has been derived either directly 

 from space, as when gases came within the influence of the earth's 

 attraction and were held by it, or from the interior of the earth from 

 which they were forced by the increasing heat produced by com- 

 pression as well as by the pressure itself. When the water vapor 

 condensed to form rain, streams began to cut down the land, under- 

 ground waters began their work of solution and deposition, the de- 

 pressions of the land were filled with water, and the earth's surface as 

 we now know it began its long series of transformations. 



Nebular and Planetesimal Theories Contrasted. — The nebular 

 and planetesimal theories differ in a number of fundamental features. 

 Under the former, the earth was once hotter and larger than now and 

 by cooling and contraction has become smaller and solid, or nearly so. 

 Under the planetesimal hypothesis, the earth became continually 

 larger as its bulk was increased by the gathering in of planetesimals. 

 The atmosphere of the earth, according to the theory of a molten 

 globe, was heaviest at first; according to the planetesimal theory, 

 it was lightest at the beginning and grew denser as the earth increased 

 in size and mass. According to the one (nebular), we may hope to 

 find the original igneous crust of the earth ; according to the other 

 (planetesimal), a " crust " never existed, but after the appearance 

 of an atmosphere the surface was composed of lava flows, volcanic 

 ash, meteoric matter, and sedimentary deposits. 



The advocates of each of these theories agree that at present the 

 earth is essentially a solid mass more rigid than steel or glass. This 

 is shown by several lines of evidence: earthquake shocks (p. 283) 

 pass directly through the earth and travel at a rate which shows 

 that the transmitting medium is an extremely rigid substance ; ex- 

 periments have shown that enormous rigidity is imparted to such 

 substances as molten rock by application of moderate pressures and, 

 hence, even if the interior is molten it would be more rigid than steel. 



REFERENCES FOR THEORIES OF THE ORIGIN OF THE EARTH 

 Chamberlin and Salisbury, — Geology, Vol. 2, 1906, pp. 1-132. 

 Croll, James, — Climate and Cosmology. 

 Moulton, F. R., — An Introduction to Astronomy. 

 Young, C. A., — A Textbook of General Astronomy. 



