THE EVOLUTION OF I'ETROLOOICAL IDEAS. 



299 



fact, and side by side with these sire placed Ho-ures of iiiicropei^'inatitic 

 and spherulitic sti'uctures copied from l^i-ofes.sor Iddines's inenutir on 

 tlie rocks of Obsidian CliH'. (Fi_os. 1-4.) 



In the case of the aUoys th(> sph<n'ulitic stnutiiics are characteiistic 

 of I'apid cooling and the niicropeomatitic structure's of slow cooling'. 

 The mode of occurrence of the same sti'uctures in rocks is strictly in 

 accordance with this vieW. A t-omparison of the structures in the 

 two cases makes it almost inipossi])le to Ixdieve that the resend)lances 



^,,,'-rf'^''^^f^^'frf!'nr~^ 



X 



^^'^''^iiiiiiM^^ 







Figs. 1 and 3 = Sphorulitie and micropegmatitic structures in olisidian. (Alter Iddings, Vllth Ann. 



Rep. V. S. Geol. Snrv., 1885-86, pi. xv.) 

 Figs. 2 and i = .Similar structures in euteetic alloys; from micmphotograplis by Stead. •_• ^ \ simple 



spherulite in the euteetic of lead atid antimony;" 4 = Micropegmatitic structure in magnolia-metal 



(lead, 80 por'cent; antimony, 15 per cent; and tin, 5 per cent). 



are mereh^ accidental, and, if not, they point to the conclusion that 

 micropegmatite is an euteetic compound. 



From this i)oint of view it becomes of interest to determine the 

 melting- point of fused micropegmatite. This was kindly done for me 

 i)y Professor Joly by observations with the nieldometer. lie found 

 that fused micropegmatite melted .somewhat more i-eadily than ortho- 

 clase, but less readily than fus(>d orthoclase. These observations do 

 not support the euteetic^ hypothesis, but they <;an scarcely be .*;aid to 



«The outlines of this splionilite ai-e soiuewhat too shari)ly drawn. 



