88 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
The rock analyzed, as is shown by its composition and also by its 
physical and mineralogical characteristics, is a typical basalt, with 
perhaps a slightly higher percentage of silica than is normal to such 
rocks. The ratio of acids and bases present, as I am assured by Prof. 
E. D. Campbell, of the University of Michigan, an expert metallur- 
gist, is such as to favor easy fusibility. From analogy with the com- 
position and behavior of the slag produced in iron smelting furnaces, 
the fusing points of which are known, the rock would probably melt 
at about 2,250° F., and yield a highly liquid product, which on 
cooling would pass slowly from a liquid to a solid state through an 
intermediate highly viscous condition. This opinion was expressed by 
Professor Campbell without a knowledge of the physical character- 
istics of the lava, and serves to increase my confidence in the deduc- 
tions made from field observations in reference to the highly liquid 
condition of the lava when extruded and its slow passage through an 
intensely viscous condition before final consolidation. The gradual 
change from a plastic to a rigid state was no doubt prolonged, owing 
to the slow rate of cooling incident to a large mass of fused material, 
but this is not of necessity the sole cause. In this connection it may 
be sugge&ted that the ready fusibility of a lava is not entirely depend- 
ent on the presence of a high percentage of basic material, as is com- 
monly stated, a basic lava usually being considered easily fusible and 
an acid lava (one rich in silica) refractory; neither does the behavior 
of a rock in this respect depend necessarily on the fusibility of the 
individual mineral present, as claimed by J. D. Dana, but rather on the 
ratio of the constituent acids and bases. With lavas, as with furnace 
slags, an excess of either acid or basic material might produce difficult 
fusibility, while an intermediate condition in which the acids and bases 
were adjusted to one another in certain definite ratios would produce 
an easily fusible compound. In the case of furnace slags, as I am 
informed by Professor Campbell, the time required for the passage 
from a liquid to a solid condition, or the time required for the material 
to "set," is regulated not only by the rate at which heat is lost, but 
by chemical composition. Slags rich in silica pass from a liquid to a 
solid state, or set much more quickly than basic slags, the conditions 
favoring the escape of heat being the same. Applying this principle 
to molten lavas we would expect highly acid lavas to congeal more 
quickly than basic lavas, even though their temperatures at the time 
of extrusion and other conditions were the same. As is well known, 
this agrees with what is found in nature, since streams of acid lavas 
have frequently come to rest and terminated with precipitous frontal 
slopes on steep gradients, which has not been observed in the case of 
basic lava. To be sure, this is not a crucial test, since the acid lavas 
in general require a greater degree of heat to cause fusion than the 
more basic varieties, and on reaching the surface may not have been 
