120 



Tllli; CRYSTAL FALLS IRON-BEARING DISTRICT. 



wliieh in t-ross .seftion give elliptical figures, Init whose iudeteriniiiate down- 

 ward extension shows them to be columns. The rounding of tlie columns, 

 which were presumably originally prismatic, he ascribes to dynamic action- 

 He also suggests that ellipsoidal masses could result from a similar dynamic 

 modification of a mass of lava parted into shorter prisms, or even 

 ellipsoids. 



In the description of the eruption at Santorin, Fouque^ mentions a 

 viscous lava exuded in the form of a mass of blocks. These blocks, tum- 

 bling over one aiiother as the mass is pushed from behind, have accumulated 

 in a rough pile, PI. XII. Fouque climbed these piles of block lava shortly 

 after their production, and noticed the breaking off of pieces from the sides, 



due to the cooling and contraction of 

 the individual blocks.^ 



In general this character agrees 

 well with that of the aa la^sa of Hawaii, 

 as described by the late Prof. J. D. 

 Dana.^ He describes the formation of 

 the blocks as due to the slow for- 

 ward movement and contemporaneous 

 The surface contrasts with the ropy 

 The aa is as a rule compact as 



-Rt^prodnction of illustratiou of aa lava, after 

 Dana (Cliaracteristics of Volcanoes). 



breaking up of the viscous lava, 

 surface of the more liquid pahoehoe. 

 compared with the pahoehoe, though the exterior "is roughly cavernous, 

 horribly jagged, with projections often a foot or more long that are bristled 

 all over with points and angles." From the illustrations of this lava (see 

 fig. 10, taken from Dana) the blocks may be seen to be, while irregular, still 

 in general distinctl}' rounded. This is the shape which viscous material 

 would naturally tend to take when subjected to the rolling action attendant 

 upon the onward motion of the stream of which they form an outer portion, 

 or in certain cases the entire thicki^ess. This is clearly shown from the 

 following quotation from Dana's description of the constitution and condi- 



' Santorin 6t des Eruptions, by F. Fouqu^: Paris, 1879, Chap. II. Compare especially Pis. VIII 

 and XIII. 



''Op. cit., p. 54. 



'Characteristics of Volcanoes, by .J. D. Dana: New York, 1890, pp. 9, 241, and Am. Jour. Sci., 3d 

 ser., Vol. XXXIV, 1887, p. 362. 



"An aa or arate lava stream consists of detached masses of lava as far as is visible from the 

 outside. The masses are of very irregular shapes and confusedly piled up to nearly a common level, 

 although often covering areas many miles long and half a mile to a mile or more wide. The size of 

 the masses in the coarser kiud varies from a few inches across to several yards." 



