532 STUDIES FOR STUDENTS 



Under contrasts it may be noted that two agents which have 

 affected largely the composition of the crust of the earth have 

 been lacking either wholly or in part in the formation of meteor- 

 ites. These agents are water and oxygen. The lack of water 

 is proved by the fresh and unaltered character of the minerals 

 found in meteorites and the absence of all hydrous minerals. 

 Thus the chrysolite of meteorites is never found serpentinized nor 

 are the pyroxenes changed to chlorite nor the feldspar to kaolin. 



Further, zeolites, micas, epidote, tourmaline and all other 

 minerals in the formation of which water and water vapor play 

 a part are entirely lacking from meteorites. 



Similarity, oxygen, at least in excess, is lacking from the 

 constituents of meteorites. Such substances as nickel-iron, 

 schreibersite, and lawrencite, which make up so large a part of 

 the composition of meteorites would rapidly have been oxydized 

 had they been exposed to the action of oxygen as it occurs 

 upon the earth. The silicates of meteorites are however oxydized 

 compounds which show that oxygen is present to some degree 

 in space. 



Again, as noted by Cohen,' the important rock-forming min- 

 erals of the crust of the earth are either lacking or play an 

 insignificant part in the formation of meteorites. Such are 

 quartz, orthoclase, the acid plagioclases, the micas, the amphi- 

 boles, leucite, and nepheline. Vice versa, the chief mineral con- 

 stitutents of meteorites occur in but insignificant amount upon 

 the earth. Such are nickel-iron, the orthorhombic pyroxenes 

 and chrysolite, while such compounds as schreibersite, cohenite, 

 lawrencite, oldhamite, daubreelite and troilite rarely or never 

 occur terrestrially. Looked at quantitatively then it may be said 

 that terrestrial rocks abound in free silica, lime, alumina, and 

 alkalies, while meteorites abound in iron, nickel and magnesia. 

 Whether these quantitative differences would be maintained if 

 the constitution of the earth as a whole could be compared with 

 that of meteorites, is, as hinted at the beginning, doubtful. 



Oliver C. Fakrington. 



'Op. cit., p. 323. 



