414 G. p. MERRILL ON METAMORPHISM IN METEORITES 



they grow less perfect, more highly altered, as they pass into crystalline 

 forms. It is, therefore, suggested that the mere presence of a chondrule 

 in a meteorite, whatever its present condition, is indicative of a tuffaceous 

 origin. The clear, limpid interstitial glass, sometimes isotropic and some- 

 times doubly refracting, is considered feldspathic (maskelynite) and as 

 due to metamorphism. Further, it is shown to have been the last mineral 

 to congeal and is believed to represent the closing act in the series of 

 changes through which a stone has passed. The dark "glassy'' interstitial 

 material is considered likewise secondary, a result of a partial refusion 

 and alteration of the finer interstitial material, accompanying a secondary 

 rise in temperature. It is shown that the crushing of the individual con- 

 stituents, while unquestionably efficacious in the development of a cata- 

 clastic structure, is a minor feature and without bearing on the question 

 of the original tuffaceous nature of the stone. The metal is shown to be 

 of secondary origin and its introduction subsequent to the consolidation 

 of the stone in its present form and quite independent of the metamor- 

 phism. 



The foregoing investigations, I may state, have been carried out with 

 other ends in view than have been thus far indicated. It was felt that 

 in following up the life history of a meteorite — noting its original con- 

 dition and the changes that have taken place — some light might be 

 thrown on its possible source and subsequent wanderings, particularly as 

 we are able to realize the conditions under which the changes have been 

 brought about. To illustrate : The chondritic meteorites have been 

 shown to be a result of explosive volcanic activity. Now, vulcanism is 

 essentially a surface phenomenon; but the meteorites invariably carry 

 unoxidized, but easily oxidizable, materials — chlorides and metals. Their 

 formation, on the assumption that these are primary, must, then, cer- 

 tainly have taken place in a dry and oxygen-free atmosphere. Our earth 

 as the maternal source of meteorites is, then, at once ruled out, unless 

 their birth be relegated to that far period of early youth when no atmos- 

 phere as we now know it existed. The rest of the story is not yet so clear. 

 The iron, as I have stated, is secondary and probably owes its reduction 

 to metallic form to the influence of hydrogen. Where such an atmosphere 

 exists is for the astronomer to tell us. The fact that the stones show the 

 effect of heat and rapid cooling may, perhaps, be accounted for on the 

 cometary hypothesis — that their brief stay in the proximity of the sun 

 was followed by so raj)id a retreat as to prevent a complete recrystalliza- 

 tion of the fused material. 



If the investigation fails to point to a complete solution of the problem, 

 it may at least serve a useful purpose in limiting the range of hypotheses. 



