B. A. Daly — Mechanics of Igneous Intrusion. 275 



sions. Mallet and others have shown reasons for scepticism as 

 to the accuracy of Bischofs early work in fusion, while Gil- 

 bert pointed out the patent error of Mallet's own method of 

 criticism.* Forbes found a net expansion of 64 per cent in 

 the passage of basalt at 20° 0. to glass at the same tempera- 

 ture, but, however accurate, his result is not here of great 

 value as he has given no information as to the degree of vesicu- 

 larity or crystallinity of the original rock.f These two char- 

 acters are so important that the further analysis of experiments 

 older than those of Barus, had best be concerned only with 

 those in which only noncrystalline, plutonic rocks or allied 

 crystalline schists were employed in fusion. The most reliable 

 of these known to the writer were made by DelesseJ and by 

 Cossa.§ Delesse made in all forty-seven fusions, but many of 

 them must, for our present purpose, be discarded because of 

 the vesicularity of the fused products. He measured the densi- 

 ties of the various rocks at normal atmospheric pressure and 

 temperature, and those of their respective glasses under similar 

 conditions. The most reliable of these determinations are 

 entered with those of Cossa in Table II. 







Table II. 











Spec. grav. 



Spec, g'rav. 



Net decrease 



Authority. 



Eock type. 



of rock. 



of glass. 



in density. 



Delesse 



Granite 



2-730 



2-450 



10-26$ 







2-623 



2-360 



10-03 







2-684 



2-423 



9-72 







2-680 



2-427 



9-44 







2-751 



2-496 



9-27 







2-700 



2-447 



9-37 







2-660 



2-425 



8-84 







2-643 



2-478 



6-24 



Cossa 



Syenite 



2-710 



2-430 



10-33 



Cossa 



Quartz dior 



ite ' 2-667 



2-403 



9-90 



Delesse 



Diorite 



2-921 



2-679 



8-29 







2-799 



2-608 



6-82 







2-858 



2-684 



6-09 



Delesse 



Gabbros 



3-100 



2-664 



14-06 







2-898 



2-641 



8-87 



Delesse 



Gneiss 



2-821 



2-625 



6-95 



If, now, we bring the averages of the last table into com- 

 parison with the results of Barus on diabase, we have Table III : 



*Kep. on the Geol. of the Henry Mts., p. 76, Washington (1887). 



| Chemical News, Oct. 23, 1868. 



{Bull. Soc. Geol. France (2), iv, 1380 (1847). 



§ Zirkel, Lehrb. d. Petrographie, i, 681 (1893). 



Am. Jour. Sci. — Fourth Series, Vol. XV, No. 88. — April, 1903. 

 19 



