260 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



vapour and gases which escaped from a lava stream at the 

 surface, Scrope formed the opinion that eruptive phenomena 

 might be traced to the mobility of the lava. According to his 

 observations, lava, as it issues from a volcanic vent, very 

 seldom has the appearance with which we are familiar in a hot 

 mass of iron or glass, but is usually in a viscid, seething 

 condition, impregnated with elastic vapours, and enclosing 

 many crystallites which move freely in the surrounding fluid 

 in virtue of the passage of the vapours through it. As the 

 vapours explode and escape, the motion of the mineral 

 constituents is impeded and the lava solidifies. Scrope 

 applied this theory to subterranean lava. He supposes a fused 

 rock-mass saturated with water, under pressure of super- 

 incumbent solid rock ; then the pressure being the same and 

 the temperature raised, or the temperature being the same and 

 the pressure relaxed, the water will pass into the condition of 

 vapour, and a certain amount of heat be made latent. The 

 crystalline constituents of this subterranean magma are 

 separated by the elastic vapour, the lava swells and passes into 

 a fluid condition. The degree of liquidity in the whole mass 

 was thought by Mr. Scrope to depend chiefly on the weight of 

 the mineral constituents and the fineness of the crystals. If 

 the subterranean lava be horizontally extended, the compressed 

 vapours, in trying to escape, press the lava against the upper 

 strata, cause earthquakes, and finally fissures into which the 

 seething lava flows. If the fissures widen towards the interior 

 of the earth, the rising lava forms dykes, and as these narrow 

 towards the earth's surface, they strengthen the crust ; but if, 

 on the other hand, the fissures are wider in the upper horizons 

 of the crust than in the lower, they remain -partially open, and 

 form relatively weak parts in the earth's crust, readily liable to 

 renewed eruptions. 



Scrope endeavoured to explain all the phenomena associated 

 with volcanic eruptions upon the basis of the above theory. 

 In favour of it, he noted the periodicity in eruptive activity ; 

 how after each eruption, when presumably the fissures have 

 been blocked with rock-material, a period of rest ensues, but 

 when the vapours have once more accumulated in the deep 

 volcanic magma, the old vent again bursts open or a new 

 orifice forms. In the case of land volcanoes, the ejected 

 products of successive outbursts surround these orifices with 

 the characteristic circular or elliptical form. The particular 



