Van Hise — Enlargements of Hornblende Fragments. 231 



Also, if the ring has been for some time magnetized in one 

 direction the temporary magnetism in this direction is less than 

 in the opposite by an amount reaching twenty per cent in 

 some cases, thus presenting an analogy to the so-called 

 "fatigue" of metals under strain.* 



The curves, however, serve to show that at the first all the 

 magnetism is temporary. The permeability rises to a maxi- 

 mum at about the same place as that of total magnetization, 

 then falls less rapidly and approaches the total, though it is 

 evident that it can never reach it, since the permanent mag- 

 netism cannot be less for a high magnetizing force than for a 

 lower at any given temperature. 



It is to be noticed that the magnets used have no free poles. 

 Hence the transient nature of the magnetism is due to the 

 structural properties of the material and not to any demagnet- 

 izing influence of the poles. 



Akt. XXX. — Enlargements of Hornblende Fragments; 

 by C. R. Van Hise. 



[Published by permission of the Director of the U. S. Geological Survey.] 



The nature of the process or processes by which frag- 

 mental rocks have become indurated is a question of great 

 interest. The enlargement of grains of quartz, as first 

 described by Sorby,f in 1880, has since been shown:}: to explain 

 in large measure the consolidation of quartzose sandstones to 

 quartzites. This enlargement consists in the deposition of 

 silica about rolled grains of quartz, the new silica being opti- 

 cally continuous with the old grains, or in other words form- 

 ing with each of the original fragments a single quartz 

 individual. The secondary growth of rolled grains of feldspar 

 in feldspathic sandstones in a corresponding manner, i. e., the 

 addition of feldspar to rolled grains of feldspar, the two being 

 parts of the same individuals, was also announced by me some 

 time since, § since which time the study of many sections of 

 feldspathic sandstones from various localities has made it cer- 

 tain that fragmental particles of feldspar frequently enlarge by 

 renewed growth just as do grains of quartz. It would seem 

 that the enlargement of feldspar fragments plays an important 

 part in the consolidation of many feldspathic fragmental rocks. 



* Righi has observed the same effect (Beibl. vol. v), and it is also similar to the 

 observation of Fromme in the case of repeated magnetization (p. 223). 



f Proc. Geol. Soc. London, 1880, p. 62. 



X R. D. Irving, Am. Jour. Sci., June, 1883. R. D. Irving and 0. R. Yan Hise 

 in Bull. U. S. Geol. Sur., No. 8, 1884. 



§ Am. Jour. Sci., Ill, xxvii, May, 1884. 



