276 B. A. Daly — Mechanics of Ig 



neous Intrusion, 







Table III. 









1. 



2. 



3. 



4. 



5. 



Sp. gr. of 







Net de- 



Net in- 



rock molten 



Sp. gr. of 



Sp. gr. of 



crease in 



crease 



at 1400° C, 



rock at 



glass at 



density, 



in vol., 



cal. from 



Rock type. ca 20° C. 



ca 20° C. 



rock to 



rock to 



Barus's fu- 







glass. 



glass. 



sion curve. 



Diabase of Barns 3-0178 



2*717 



10*00$ 



11-2 # 



2*523 



A v. gabbro of Delesse 2*999 



2*642 



1157 



13*1 



2*507 



Av. diorite " 2*859 



2*657 



7*07 



7*6 



2*390 



Q. diorite of Cossa 2*667 



2*403 



9*90 



11-1 



2*229 



Syenite of Cossa 2*710 



2*430 



10*33 



11-5 



2*266 



Av. granite of Delesse 2*684 



2*438 



9*16 



' 10*0 



2*243 



Av. of all above 





9-67 



10*7 





Gneiss of Delesse 2*821 



2*625 



6'95 



7-5 



2-358 



The conclusions immediately derivable from columns 3 and 4 

 are : first, that the results of Barus, Delesse and Cossa on the 

 whole agree closely and are mutually corroborative ; secondly, 

 that the net expansion increment of diabase in passing from 

 the noncrystalline to the glassy state (at one atm. pressure), is 

 nearly the average for all plutonic types ; thirdly, that the 

 agreement of Barus and Delesse is specially close when the 

 two observers used rocks of nearly equivalent chemical com- 

 position ; fourthly, that, from the similar behavior of plutonic 

 types within the limits investigated, it is fair to consider it 

 highly probable that all the types would be characterized by 

 nearly the same rate of expansion in passing from the non- 

 crystalline to the molten state at 1400° C, although experi- 

 mental data be now lacking for testing the justice of that 

 inference. (Col. 5 contains the values of the corresponding 

 specific gravities calculated from Barus's fusion curve as the 

 standard) ; and, lastly, that, as expected, decreasing acidity 

 means increasing density of the molten magma at 1400° C. 



In view of the disturbing influence of vesicularity and of 

 variable crystallinity in his lavas, it appears that Delesse's table 

 showing that thermal expansibility is strongly affected by 

 basicity (fusibility), is unreliable.* In fact, so closely accordant 

 are the various rock-types referred to in Tables II and III, 

 that we may not expect serious error in applying Barus's 

 accurate expansion curve to plu tonics generally. The writer 

 has therefore constructed Table IY, in which col. 1 represents 

 the range of specific gravity found in the different chemically 

 analyzed types noted in Rosenbusch's Elemente der Gesteins- 

 lehre. Col. 2 represents the range of specific gravity (calculated 

 from the fusion curve) in the same rocks molten at 1400° C. and 

 at normal atmospheric pressure. Col. 3 gives the specific grav- 

 ities of the same rocks supposed to follow the law of normal 

 *Op. (dt., p. 1390. 



