34 Day, etc. — Determination of Mineral 



work on the silica minerals,* the fact that quartz in pegmatites 

 is frequently found to have crystallized in the neighborhood of 

 575° may be connected with the opening of fissures in the cool- 

 ing granitic rock by the sudden volume change taking place 

 in quartz which has already crystallized. 



But the significance of this point is not alone in the sudden- 

 ness or the amount of the volume change in the mineral, but 

 also in the small heat effect which accompanies it. The crys- 

 tallization of a magma may be accompanied by a considerable 

 volume change, yet the amount of latent heat given out dur- 

 ing crystallization might delay its progress to such an extent 

 that the volume change of the mass as a whole would proceed 

 relatively slowly. But the amount of heat accompanying the 

 quartz inversion is too small to seriously affect the rate of 

 cooling of the rock, so that the volume contraction, at this 

 temperature (575°), of a rock containing much quartz would be 

 sudden and considerable. 



In his papers on " The mechanics of igneous intrusion,"f 

 Daly suggests that an important cause for the cracking off of 

 blocks of rock in contact with a liquid intrusive mass is to be 

 found in the unequal expansion due to sudden heating. The 

 experiments on quartz and granite in the preceding pages 

 emphasize rather the much greater importance which must be 

 ascribed to the sudden volume change accompanying the transi- 

 tion of quartz at 575°, or to other similar transitions. Their 

 effect in shattering the rock is much greater than the effect of 

 ordinary thermal expansion even in a region of steep tempera- 

 ture gradient. The result, however, would not be the break- 

 ing off of large blocks which, according to Daly's u stoping 

 hypothesis," must then sink in order to be assimilated at 

 greater depths, but rather the formation of a shattered porous 

 material which would be in the best possible condition for 

 immediate assimilation in situ. 



Moreover, the capillary penetrating power of liquid diabase 

 into a porous mass of this character is very considerable. Dia- 

 base melted in a crucible of alundum (granular fused alumina) 

 came through the sides of the crucible, and rose by capillarity 

 in the walls like water in filter paper. We have even found 

 numerous tiny drops of diopside on the outside walls of the 

 inverted graphite float-crucible of the apparatus of fig. 1, the 

 liquid silicate apparently having been forced through the fine 

 pores of the graphite by the small pressure of the surrounding 

 molten tin. 



Daly has assembled in his paper on " the secondary origin 



*C. N. Fenner, this Journal, xxxvi, 379, 1913. 



\R. A. Daly, this Journal, xv, 269-298, 1903; xvi, 107-126, 1903 ; xxvi, 

 17-50, 1908. 



