26 Field Columbian Museum — Geology, Vol. i. 



In specific gravity the majority of aerolites range from 3.00 to 

 3.80, being on the whole heavier than terrestrial rocks of the same 

 nature on account of the greater quantity of metallic constituents. 



Viewed as to structure the greater number of aerolites are found 

 to be made up chiefly of little spheres, varying in size from those as 

 large as a cherry to those only visible under the microscope. These 

 are called chondri from the Greek %ovdpo? f a ball, and meteorites pos- 

 sessing this structure are said to be chond> itic. 



The chondritic structure is often discernible by the naked eye, as 

 may be seen in the specimens of Weston (224), Forsyth (240), Pusi- 

 nsko (250), Trenzano (268), Knyahinya (284) and many others. 



When examined with sufficient magnifying power the chondri 

 can be seen to be composed of angular, crystalline fragments chiefly 

 of chrysolite or some pyroxene. See Plate VI, Fig. 1. These may be 

 present as one individual (monosomatic) or more commonly of several 

 (polysomatic). 



An eccentric fan-shaped chondrus made up of radiating fibres 

 of enstatite is a very unique and characteristic form. One such may 

 be noted in the section of the Simbirsk meteorite, shown in Plate VI, 

 Fig. 1. Other arrangements of the grains or fibres which may be 

 noted are concentric, reticulated and radiated. 



The chondrus is frequently enclosed in a shell of metallic grains 

 which gives it a distinct outline and separates it from the ground mass. 

 This is illustrated in Plate VI, Fig. 2. 



The conditions which have brought about the formation of these' 

 chondri are not well tinderstood 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 pre-existing rock which by oscillation and consequent 

 attrition obtained a spherical form. Sorby* has regarded them as 

 produced by cooling and aggregation of minute drops of melted 

 stony matter. Tschermakf considers their origin similar to that of the 

 spherules met with in volcanic tuffs, which owe their form to pro- 

 longed explosive activity in a volcanic throat, breaking up the older 

 rocks and rounding the particles by constant attrition. 



Different views are, however, set forth by BrezinaJ and Wads- 

 worth§, who believe that the chondri have been produced by rapid 

 and arrested crystallization in- a molten mass. 



*Geol. Mag. 1865 (t) ii, 447. 



tPhil. Mag. 1876 (5) i, 497-507. 



JDie Meteoritensammlung in Wien, 1885, p. 19. 



§LithoIogical Studies, p. no. 



