THE ORIGIN OF THE EARTH. 29 



taken place in small masses of matter projected into space in the 

 molten condition, and the view that meteorites are formed directly 

 from lavas shot into space by volcanic or other explosive action, as 

 from a sun, a planet, or the moon, is, therefore, unsatisfactory. Equally 

 adverse to this view is the extraordinary fact that certain classes of 

 meteorites are formed chiefly of hydrocarbons which are volatile at 



Fig. 7. — Section of the Mount Joy meteorite, nickel-iron, showing brecciated struc- 

 ture. Field Columbian Museum. (After Farrington.) 



moderately high temperatures, and are readily combustible. These 

 hydrocarbons seem prohibitive of high temperatures at all stages of 

 their history, and it is a marvel that they should survive the transit 

 through the atmosphere; but this is probably due to the fact that the 

 meteorites were excessively cold when they entered the atmosphere 

 and, during the brief time of transit, were only superficially consumed, 

 while their interiors remained cold, as the interiors of meteorites are 

 not infrequently found to be, immediately after their fall. 



Igneous processes on the earth give rise to magmatic differentiation 

 resulting in a familiar series of minerals which make up large portions 

 of the crystalline rocks of the earth's surface. So also, weathering 

 and solution remove more of the basic than of the acidic constituents 

 of crystalline rock, and when the residue is metamorphosed, a similar 

 series of minerals arises. Among these are quartz, orthoclase, the 

 acid plagioclases, the micas and the amphiboles, a group absent from 

 the meteorites. Their absence suggests that, in the parent body, mag- 



