BASIC VOLCANICS OF HEMLOCK FOEMATIOX. 121 



tiou of the aa stream when iu motion : ^ " (1) A mass of rougli bhx'ks outside, 

 precisely hke the cooled aa stream; (2) the motion extremely slow, indi- 

 cating a semifluid condition beneath; ... (5) the blocks of the upper part 

 of the front, as the stream creeps on, tumbling down the high slojje, owing 

 to retardation at bottom from friction, and thus a rolling action in the front 

 part." 



Dana describes the gradation of pahoehoe into aa lava. He writes, "a 

 lava stream may change from the smooth-flowing or pahoehoe condition to 

 the aa and back again to the smooth-flowing."" 



Platania^ describes from Aci-Trezza and Aci-Castello basalts with 

 globular structure. The interspaces between the globes are filled with silt, 

 or silt and tuff", and the exterior of some of these globes presents a thin vit- 

 reous cracked crust (cf. p. 117). These globular basalts are apparently but 

 a modification of the block or aa lavas described by Fouqu^ and Dana, in 

 which the separate portions of the laA^a liaA'e assumed a sufficiently rounded 

 character to be called globes. However, Platania's further descriptions show 

 this term to be clearl}" inapplicable unless the word "globe" is used with 

 considerable latitude. 



The Santorin block lava, tlie Hawaiian aa lava, and the Aci-Castello 

 globular lava are all products of a slowly-flowing comparatively viscous 

 mass. They will in the further description be included under the general 

 term " aa lavas," as this is the most common form of occurrence of such 

 viscous lavas. 



The ellipsoidal basalts of the Crystal Falls district appear to be com- 

 parable to the Hawaiian aa lava and block lavas of the kind described b}' 

 Fouque'. The lavas have subsequently been exposed to great pressure and 

 are considerably altered. The most obvious character of these masses, their 

 rounded outline, is believed to be due to considerable extent to the onward 

 motion of the stream as described by Dana. 



Contraction caused by cooling, accompanied by falling off of fragments 

 from the outside, as observed by Fouque* in the Santorin block lava, would 

 also tend to round blocks which were originally ang-ular. (PL XL) In 



' Characteristics of Volcanoes, by J. D. Dana, New Yorli, 1890, p. 242 ; and Am. Jour. Sci., 3cl ser., 

 Vol. XXVI, p. 100. 



= Am. Jour. Sci., 3d ser., Vol. XXXIV, p. 363. 



■'Geological notes of Acireale, by Gaetano Platania : The Southern Italian Volcanoes, H. J. 

 Johnstou-Lavis, editor, Naples, 1891, Chap. II., p. 41. 



^Op. cit., p. 54. 



