54 STUDIES FOR STUDENTS 



are however plainly polygenic. An etched section of the Mt. 

 Joy meteorite for example shows the mass plainly to be made 

 up of irregular iron fragments. The structure of each fragment 

 as shown by its etching figures is sui generis, and indicates an 

 independent origin. The iron of Zacatecas is likewise made up 

 of individual grains the size of a hazelnut to that of a walnut. 

 These are separated by areas of troilite. 



Many of the iron-stone and stone meteorites are monogenic 

 but more are made up of two or more different kinds of rock. 

 To draw the dividing line between the monogenic and polygenic 

 meteorites of the last two classes is not an easy task and the 

 opinion of no two observers would probably be the same in regard 

 to it. The meteorite of Stannern for instance was described by 

 one observer as crystalline and by another as clastic. Tscher- 

 mak, who has given the matter profound study, is disposed to 

 regard practically all stone meteorites as of a tuffaceous or 

 clastic character while Wadsworth after examining many meteor- 

 ites concluded that none which he had examined could be con- 

 sidered "fragmental in the sense of consolidated cold masses 

 joined together." The present writer can only state that in his 

 opinion some stone meteorites are so uniform in character that 

 crystallization from a single magma is indicated while on the 

 other hand many meteorites have a clearly brecciated and tuf- 

 faceous character showing them to be polygenic. 



Structures of terrestrial origin to be eliminated. — In all study 

 of the structure of meteorites with a view to learning their pre- 

 terrestrial history, care should be taken to eliminate all phe- 

 nomena of terrestrial origin. Thus the crust of meteorites and 

 their surface markings are usually considered, and without 

 doubt properly, to be produced during the passage of the mass 

 through the earth's atmosphere. The possible effects on the 

 interior of a meteoric mass, of heat developed by such passage 

 should also be borne in mind in study. Again the force of impact 

 with which a meteorite strikes the earth is often very great. 

 It should be considered whether such a blow might not give rise 

 to phenomena of internal movement within the mass. Again 



