THE EXPLANATION OF THE SEQUENCE. 



135 



the theory is fatally impaired. In the absence of evidence fixing the inter- 

 section here, we might have arbitrarily taken it to be at some other point — 

 at a point, too, outside of the scale of acidity within which volcanic rocks 

 are always confined, as in Figs. 1 and 2. In either of these cases the 



Fig. 1. Fig. 2 



2.60 



2.50 



40 



50 



60 



70 



80 



40 50 60 .70 80 

 rocks would have been, according to the terms of the theory, erupted 

 strictly in the direct or inverse order of their densities throughout. But I 

 believe we do possess some distinct evidence that the point of intersection 

 is rightly chosen, and that this evidence may be read in the petrographic 

 and mechanical characters of the rocks themselves. A very striking 

 characteristic of the basaltic lavas is their perfect liquidity at the time of 

 eruption and their power to flow in comparatively narrow and shallow 

 streams to great distances. It is in the basalts that this property is most 

 marked and conspicuous. Coulees only two or three hundred feet wide 

 and only twenty or thirty feet thick are usually found flowing mile after 

 mile with facility, and larger streams reach from thirty to fifty miles from 

 their orifices. Very thin sheets of basalt flow on to great distances. No 

 other rocks in streams of such small cross-sections reach distances so far 

 from their origin. And when we recall the circumstances which favor a 

 rapid cooling and solidification, this preservation of fluidity is remarkable. 

 The experiments of Bischof and Deville agree in indicating that the latent 

 heat of fusion is less in the basalts than in other rocks. The larger amount 

 of surface which these thin streams or sheets expose, the disappearance of 

 heat which is consumed in expelling in the form of vapor the included 

 water, all combine to dissipate or render latent the contained heat of the 



