180 STUDIES FOR STUDENTS 



than one of the kinds above described usually occur promiscu- 

 ously scattered through the same stone. There is no gathering 

 of them into groups according to the minerals they contain. 

 Occasionally one chondrus encloses another, and still more 

 rarely two may be joined together. Broken fragments of chon- 

 dri commonly occur in the same stone with complete chondri. 

 Two fragments of the same chondrus are, however, rarely if 

 ever found in juxtaposition. Hence there must have been con- 

 siderable separation of the fragments before consolidation of the 

 stone took place. 



Theories of the Origin of Clwndri and Chondritic Structures. — The 

 conditions which have brought about the formation of chondri 

 are not well understood, though the question has been much 

 discussed and various hypotheses have been suggested. The 

 views of earlier observers were to the effect that the chondri 

 represented fragments of preexisting rock which, by oscillation 

 and consequent attrition, obtained a spherical form. Sorby 

 regarded them as produced by cooling and aggregation of 

 minute drops of melted stony matter. Tschermak considers 

 their origin similar to that of the spherules met with in volcanic 

 tuffs which owe their form to prolonged explosive activity in a 

 volcanic throat breaking up the older rocks and rounding the 

 particles by constant attrition. 



Different views are, however, held by Brezina, Wadsworth, 

 and others, these believing that the chondri have been produced 

 by rapid and arrested crystallization in a molten mass. 



Objections to theories of the first class are to be found (i) 

 in the fact that the chondri usually have rough-knobbed surfaces 

 instead of smooth ones, such as attrition might be expected to 

 produce; (2) in the regularly eccentric form of most enstatite 

 chondri, which attrition would be likely to destroy; and (3) in 

 the fact that fragments of a preexisting rock ought to show the 

 constitution of the rock as a whole instead of specialized struc- 

 ture. Objections to theories of the second class are to be found 

 chiefly in the clearly fragmental nature of most chondritic 

 meteorites. It is in their variation from the surrounding ground 



