230 ROBERT B. SOSMAN 



as yet impossible to say. Longchambon 1 suggested that the super- 

 imposed spheroids are due to a breaking up of long liquid convec- 

 tion cells into a number of shorter ones, each with its own local 

 circulation, but there is no experimental evidence to support this. 

 7 . Irregularities of faces of prisms. — Some basalt prisms show the 

 "feather-patterns" characteristic of fractures in homogeneous 

 solids. Their occurrence points strongly to a purely contractional 

 origin. They have been observed in the joint planes of slates, and 

 have been made the subject of an interesting study by Woodworth. 2 



SURFACE STRUCTURE PRODUCED BY INTERNAL EXPANSION 



In addition to the prismatic structures produced by contraction 

 and convection or by convection combined with crystallization and 

 contraction, still another type needs to be considered, namely that 

 due to expansion. 



The accompanying photograph of a polygonal structure in a 

 cement briquette (Fig. 4) is an illustration of the formation of this 

 structure by internal expansion. This sample, which was kindly 

 furnished us by Mr. A. A. Klein, of the Bureau of Standards in 

 Pittsburgh, was made from a cement which contained free lime; 

 this by its hydration and absorption of carbon dioxide from the air 

 has expanded and destroyed the briquette. 



It is possible that the "weather-crack" structure on the surface 

 of diabase bowlders is likewise caused by internal expansion. 

 Wherry 3 has shown that there is no visible difference in texture 

 underlying these weather-cracks. Expansion of the surface by 

 hydration has been assumed as the cause of the structure; but this 

 would produce compression in the surface, accompanied by the 

 formation of shells (as indeed often occurs), whereas the "weather- 

 crack" structure is one indicating tension. It is necessary for 

 hydration to proceed into deeper portions of the rock before tension 



1 Soc. Geol. France, Compt. rend, somm., 1912, pp. 181-83; Bull., 'Kill (1913), 

 33-38. 



2 Proc. Boston Soc. Nat. Hist., XXVII (1896), 163-83. For an extended study of 

 these feather fractures in glass and metals see Ch. de Freminville, "Recherches sur 

 la fragilite; L'eclatement," Rev. metallurgie, 1914; also Mallock, Proc. Roy. Soc, A, 

 LXXXII (1909), 26-29. 



3 Loc. cit. 



