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Show THE EXPLANATION OF THE SEQUENCE. 135 the theory is fatally impaired. In the absence of evidence fixing the intersection here, we might have arbitrarily taken it to be at some other pointâ€" 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. 2 290 2.80 270 2.60 2.50 K..OKJ \â€"â- ------___ ,-------------------.---------------------.---------------- 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 |