504 Prof. Gr. Tschermak on the Formation of 



They resemble rather the spherules which are frequently met 

 with in our volcanic tuffs, such as the chondra of the trachyte 

 tuff of Gleichenberg, those of the basalt tuff of the Venusberg, 

 near Freud en thai, and more especially the spherules of olivine 

 of Kapfenstein and Feldbach in Styria. 



As regards the last-mentioned chondra*, we know them to 

 be the result of volcanic trituration, and to owe their form to a 

 prolonged explosive activity in a volcanic " throat," where the 

 older rocks have been broken up and the tougher particles 

 have been rounded by continued attrition. 



The characters of the meteoric chondra indicate throughout 

 a similar mode of formation f. We may at any rate conceive 

 the rock-masses which suffered trituration to have possessed 

 a somewhat soft texture, and thus arrive at an approximation 

 to Daubree's view J of a rock-mass which assumed a solid 

 form while rotating in a gaseous medium ; it is certain, in 

 short, that the spherules are the result of trituration. 



The spherules are sometimes microscopically small, have 

 sometimes the size of a millet-seed ; others may be as large 

 as a cherry or a small hazel-nut ; but they arc not numerous. 

 The tufaceous chondra of our volcanic rocks vary in size from 

 that of a hazel-nut to that of a head. If from this disparity 

 we may form any conception of the different magnitudes of 

 the areas of activity in which they have been produced, we 

 must seek in numerous minute volcanic fissures for the source 

 of the meteoric tuffs. 



The latter are peculiarly characterized as containing no 

 trace of a slag-like or vitreous rock, nor enclosing distinct 

 crystals in the matrix ; in short they exhibit nothing which 

 their formation from lava would lead us to look for. All that 

 is to be seen in them is the triturated product of a crystalline 

 rock. 



Some of the tufaceous meteorites bear evidence of a later 

 modification wrought by heat : such are the stones of Tadjera 

 and Belgorod §. Others, again, exhibit phenomena which 

 can only be explained on the theory of their having under- 

 gone a chemical change subsequent to their formation. We 

 not unfrequently find, for example, in the meteorites of Mezo- 

 Madaras and Knyahinya concentric aggregations of metallic 



* They must not be confounded with the volcanic bombs which are 

 composed of lava. 



t Reichenbach considered the chondra to be small meteorites. Here, 

 however, the aim is merely to form a conception of a planetary form 

 of meteorites. 



X G. A. Daubree, Journal des Savants, 1870, 38. 



§ Sitzungsb. Akad. Wiss. Wien, vol. lxx. Abth. i. November Heft. ; and 

 S. Meunier, Compt. Bend. vol. lxxii. p. 839. 



