424 Reports and Proceedings — 



produced at different and successive ages. Many of tliem were 

 fractured and displaced in consequence of movements of the mass of 

 rock traversed by them ; and these dislocations are regarded by the 

 author as indicating the vast extent and force of the internal move- 

 ments, due principally to gravity, which are constantly taking place 

 in the mass of volcanic cones. These movements greatly influence 

 the position of the dykes, and render it difficult to ascertain that 

 which they originally occupied. The dykes thin out at various 

 heights, and their superior and northern terminations were fouad not 

 to reach the existing surface, notwithstanding the amount of denuda- 

 tion that has taken place ; and hence the author concludes that they 

 never reached the surface of Somma, when it was the wall of an 

 active volcano. The author further indicated a process by which beds 

 or plates of lava descending the slopes of a volcano may change their 

 direction, and becoming imbedded in the detritus accompanying or 

 following them, may, to a greater or less extent, simulate dykes, 

 although in this case the two sides of the plate will present the dif- 

 ferences always seen in the upper and under surfaces of a bed of lava. 

 The orientation-lines of five or six of the observed dykes were said to 

 pass approximately through the axis of the cone of Vesuvius, but 

 all the rest presented great diversities, and some, when prolonged, 

 would not touch the cone at all. In making a lithological examina- 

 tion of the dykes of Somma, the author directed particular attention 

 to the position of the elongated air-bubbles found in the material of 

 each dyke, considering that the direction of the longest axis of these 

 bubbles would indicate the flow of the material when in fusion. 

 He stated that on the whole the long axes of the bubbles are nearly 

 horizontal or pointing at moderate angles upwards in directions 

 very nearly parallel to the plane of the dykes at the place where 

 they occur. Hence he inferred that the dykes were filled by injec- 

 tion not from below but nearly horizontally. The author further 

 referi'ed to the mineralogical characters of the materials of Mie 

 dykes, and stated that they are not all composed of leucitic lava ; 

 he also mentioned the occurrence of cross columnar structure in 

 some of the larger ones. After referring to the differences observ- 

 able in the physical condition of the two surfaces of some dykes, the 

 author proceeded to consider the mode of the fissures which, when 

 filled, constitute volcanic dykes. He maintained that the production 

 of a fissure and its filling with molten matter must have been 

 simultaneous and due to the same cause, namely, the hydrostatic 

 pressure of the liquid lava more or less filling the crater, the 

 pressure originating the fissure into which the pressing liquid at the 

 same time enters ; a fissure thus produced and filled will always be 

 widest near the crater, so that, if the material of the cone were 

 perfectly uniform, the dykes produced would be wedge-shaped. But 

 from the absence of this uniformity and other causes, fissures com- 

 menced at the interior and propagated into the mass of volcanic 

 cones can rarely be uniformly distributed round the crater or pro- 

 duced in regular vertical planes in a truly radial direction. Hence 

 the author concluded that it is unsafe to attempt to fix the posi- 



