VII. 



Notes on Pipernoid Structure of Igneous 



Rocks. 



MOST geologists are acquainted with a peculiar rock, much 

 employed as a building stone in Naples, that occurs near that 

 town at a village called Pinaura, and known by the name of 

 " Piperno." What the origin of this name may be, I do not pretend 

 to know, but it much resembles the word pepevino, a true though 

 peculiar tuff, derived from a sub-lacustrine eruption of the Colli 

 Albani, near Rome, but entirely different in character. 



For all practical purposes, this rock may be called a rather basic 

 trachyte, but not uniform in structure. It is of a clear grey colour, 

 interrupted by innumerable flackers,^ when seen in section lying 

 parallel to the surface on which the rock bed rests. These, when 

 isolated, are seen to be flattened cakes of an original pasty material, 

 rather longer in one definite axis with striae parallel to the same, and 

 of such a nature as to show that shearing occurred in the viscous 

 material between the upper and lower surface of each cake. Chemi- 

 cally and microscopically, these two constituents are identical, except 

 that the black flackers, or cakes, are more compact, exhibit true 

 fluxion structure in harmony with their planes of shearing, and are 

 much more vitreous ; in fact, they present all the characters of a 

 lava. The grey material is very spongy, but does not present true 

 vesicular structure, extending beyond the minute particles of which it 

 is composed, and, in fact, exhibits all the characters of a tuff — that is, 

 a consolidated volcanic dust. 



Piperno occurs in one or more beds in the neighbourhood of 

 Pianura, Soccava, and Monte Spina. Of late years I have paid much 

 attention to this rock, and I have been successful in working out the 

 stratigraphy and physical history in detail of the Phlegrean Fields 

 and the Campanian plain. The principal conclusions that I have 

 come to have been published in the Reports Brit. Assoc, since 1884. 

 In these researches I was able to show that we could trace the piperno 

 into the grey tuff which covers several hundreds of square miles of the 



1 This word is used by cabinet-makers to denote the marlcings due to the cutting 

 the medullary rays in an oblique manner, which resemble the graining of piperno. 



