110 Z. V. PirssoH — AvtifiGial Lava-Flow 



ill the direction of this cleavage. It is not singular in this 

 resjHH't, for most minerals tend to he elongate, ov coluninar, in 

 the direction of pronounced cleavages. In the direction normal 

 to the cleavao-e faces the molecular network has a lesser 

 amount of coliesive attraction than in otlier directions and 

 hence the cleavage. We may therefore imagine that during 

 the process of growth the amount of molecular tension or pull 

 exerted by the crystal upon the unorientated molecules in the 

 magma is greater toward the end face than toward the prism, 

 or cleavage, faces. Hence the supply of molecules u[)on it is 

 more rapid and the crystal grows faster in this direction. If 

 now the viscosity of the magma rises to such a degree that the 

 tension toward the side faces is not sufficient to overcome it 

 and orientate the molecules while that upon the end face is 

 sufficiently great, then the crystal will extend itself like a long 

 rod, or fiber, until the growing viscosity puts a stop to further 

 progress in this direction also. The crystallizing effect of the 

 fiber end would also be aided by the fact, that as the molecules 

 fall into position in the geometric network, or change from the 

 liquid to the solid state, a certain amount of heat is liberated, 

 tending to increase the mobility of the still unfixed adjacent 

 molecules and to render thein more susceptible to crystallo- 

 graphic orientation. The crystals then, like a wire with a hot 

 end, bore out into the stiffening magma in all directions, and as 

 conditions are uniform about them they cease simultaneously 

 and the globular shape results. This relation of habit between 

 viscosity and cohesive attraction is also well illustrated by the 

 wollastonite crystals in the dark glass. They extend indefi- 

 nitely along the h axis perpendicular to which there is no 

 cleavage and are also tabular to the a (100) face, parallel to 

 which the best cleavage occurs. In the fibers of feldspar in 

 spherulites in the \'olcanic rocks the elongation in the major- 

 ity of cases is j^aralle] to the clino axis, the direction of the 

 two prominent cleavages, The instances cited wliere the elon- 

 gation is parallel to the c axis* would tend to show that the 

 molecular attraction to the c (001) face in orthoclase is greater 

 than that to the h (010) face. The fact that large crystals and 

 especially Carlsbad twins are often tabular or elongate on the c 

 axis also shows this. It is generally considered that the cleav- 

 age c(OOl) of orthoclase is better than that of l> (010), and this 

 would appear to make these cases an exception to the general 

 rule. It is not clear, however, judging from the views of the 

 writers previously stated, that viscosity is the only factor to 

 consider in these cases, as it is in the Kane glass, as a delicate 

 balance between several different factors may have induced 



* Iddings, Sphemlitic Crystallization, Bull. Phil. Soc. Wash., vol. xi, p. 

 456, 1891. 



