PRISMATIC STRUCTURE IN IGNEOUS ROCKS 227 



5. Difference in composition and texture between the axis and the 

 periphery of the columns. — Obviously, no variation whatever should 

 appear in contraction columns. If the columns are due to convec- 

 tion, however, there might or might not be a differentiation, depend- 

 ing upon whether the rock crystallized practically as a unit, or 

 whether it crystallized in stages which permitted of segregation in 

 the convection cells (see p. 223). 



In 1914 Dr. H. S. Washington, of this laboratory, examined, in 

 the museum of the University of Catania, a polished section of a 

 column from one of the prehistoric basaltic flows of the Mount 

 Etna region, and observed no variation of texture across the 

 section. From their shape and manner of occurrence, these 

 columns at Etna would seem to be due to pure contraction, and 

 no variation is to be expected. 



On the other hand, evidence is not lacking in geological litera- 

 ture of what seems to be a differentiation between the border and 

 axis of some basalt columns. Scrope, in his description of the vol- 

 canoes of central France, states that "occasionally (as for example 

 at La Tour d'Auvergne, in the Mont Dore), the columns show a 

 cylinder of compact black basalt within a prismatic case of lighter 

 colour and looser texture, a segregation of dissimilar matter having 

 accompanied the concretionary action." 1 Delesse 2 made in 1858 

 an interesting comparison of the density of the interiors and 

 exteriors of a variety of columns, the results of which are shown 

 in Table III. Here again a difference between the interior and 

 exterior is indicated in some of the columns, though not in all. 

 Unfortunately the source of the samples which showed small differ- 

 ences is not stated; it may be that they are columns of the narrow 

 contraction type. Delesse took care to assure himself that the 

 differences were real and were not due to weathering of the columns, 

 but it is not impossible that the differences are really due to weather- 

 ing, since he had not the modern microscopic facilities for examining 

 the individual minerals in thin section. 



1 Volcanos, 1862, p. 100. In speaking of "concretionary action" Scrope seems to 

 be referring to the rather vague hypothesis of columnar structure which prevailed at 

 the time (see note, p. 216). 



2 Delesse, "Variations dans les roches se divisant en prismes," Compt. rend., 

 XLVII (1858), 448-50. 



