3 9 ( > Geological Society. 



dykes, especially as regards the axis of the cone of Vesuvius ; to this 

 direction he gives the name of orientation. Of twenty-seven dykes ten 

 presented an approximately vertical line, whilst all the rest had a 

 sensible dip or 4i hade." The dykes are in no cases intersected by 

 coherent beds of lava ; but in one instance the top of a dyke was 

 stopped by such a bed. Many of the dykes bifurcated or branched ; and 

 frequently two clykes intersected each other at considerable angles. 

 These and other circumstances prove that the dykes were produced 

 at different and successive ages. Many of them 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 indi- 

 cating the vast extent and force of the internal movements, 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 ori- 

 ginally occupied. The dykes thin out at various heights ; and their 

 superior and northern terminations were found not to reach the 

 existing surface, notwithstanding the amount of denudation that has 

 taken place ; 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 differences always 

 seen in the upper and under surfaces of a bed of lava. The orien- 

 tation-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 examination of the 

 dykes of Somma, the author directed particular attention to the posi- 

 tion 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 injection not 

 from below, but nearly horizontally. The author further referred to 

 the mineralogical characters of the materials of the 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 observable in the physical condition 

 of the two surfaces of some dykes, the author proceeded to consider 

 the mode of origin of the fissures which, when filled, constitute vol- 

 canic 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 fis- 

 sure, into which the pressing liquid at the same time entered. A 

 fissure thus produced and filled will always be widest near the crater; 



