978 A TREATISE ON METAMOEPHISM. 



CHLORINE. 



According- to Clarke's estimate of 1891 chlorine, including bromine, 

 forms 0.15 per cent of the original rocks, the hydrosphere, and the atmos- 

 phere. Of the ocean it composes 2.07 per cent. His estimate for the 

 original rocks in 1891 and 1900 is the same, 0.01 per cent. 



As to the source of chlorine the only minerals of the original igneous 

 rocks in which it is found are apatite, socialite, and the marialite molecule 

 of the wernerites. Chlor-apatite and marialite are very unimportant original 

 minerals, and therefore the chief mineral in which it is found is sodalite, of 

 which it constitutes 7.3 per cent." 



It has already been explained (pp. 789-790) that chlorine is emitted 

 from volcanoes as hydrochloric acid and to a less extent as chlorine. 

 It has been further noted that a part of this chlorine may possibly have 

 been derived from sea water; but there is reason to suppose that much 

 of it is that of the original magmas. So far as this is true we have an 

 original source for chlorine. And when it is considered that at periods of 

 regional volcanism the amount of chlorine which probably issued from 

 volcanoes was vastly greater than at the present time of local volcanism, 

 it follows as a possibility that volcanism is the most important source of 

 that element. 



In the meteorites chlorine occurs in lawrencite (FeCh). Lawrencite 

 is a volatile and readily decomposable mineral. If at the time the earth 

 stuff segregated chlorine was contributed as lawrencite, it is certain that the 

 action of water in the magmas upon this compound would produce hydro- 

 chloric acid; this- suggests a source of a part of the hydrochloric acid of 

 volcanoes. 



In the secondary rocks the amount of chlorine is very small. In the 

 mechanical sediments it is so small that it can not be estimated. It is not 

 mentioned in the analyses of the shales, and only a trace is reported in the 

 sandstones. In 843 limestones chlorine composes 0.01 per cent. The 

 sea water which was originally between the pores of the mechanical 

 sediments must have contained considerable chlorine. The absence of 

 chlorine from the mechanical sediments shows how thoroughly the sea 

 water has been squeezed or leached out in the transformations of the 

 rocks from their original forms to shales, sandstones, etc. The presence of 



"Dana, E. S., A system of mineralogy, Wiley & Sons, New York, 6th ed., 1892, p. 429. 



