396 



of Oporto to the mouth of the Douro, and on the south and the east 

 for several miles. On the gneiss, in the direction of Vallongo, reposes 

 chlorite slate, the strata of which are stated to be inclined at a very 

 high angle. The next deposit, resting conformably on the last, con- 

 sists of alternate beds of anthracite and conglomerate, derived from 

 the rocks already enumerated ; and to these strata chlorite slate again 

 succeeds, forming the range of hills called the Serra de Vallongo. In 

 this part of the series veins of metalliferous quartz occur, containing 

 ores of antimony and silver. To the east of the Serra de Vallongo 

 the chlorite slate is overlaid by a dark blue argillaceous schist. Be- 

 yond this point the author's observations did not extend. 



An Essay " On the Curvilinear Structure of Lava/' by Signor 

 Monticelli of Naples, was afterwards read. 



The object of the author is to attract the notice of geologists to a 

 peculiarly beautiful and symmetric arrangement which he has ob- 

 served in the lava of La Scala, one of the largest and most ancient 

 currents of Vesuvius. The existence of numerous perpendicular and 

 horizontal fissures which traverse this lava, and sometimes give it the 

 appearance of regular stratification, was described by Breislac ; and 

 the same observer noticed its tendency to split, under the hammer, 

 into irregular prisms of an hexagonal figure. But a far more sym- 

 metric arrangement was recently discovered in a grotto opened by 

 the workmen in quarrying the lava. The walls of lava bounding this 

 grotto were distinctly curvilinear ; several distinct curvilinear strata 

 were traced with their seams parallel to each other ; and the grotto 

 itself, decreasing in height and width towards either extremity, pos- 

 sessed the form of an ellipsoid. The author describes another similar 

 arrangement of the lava at the same locality, consisting of not fewer 

 than fourteen successive, parallel strata of a spherical form, arranged 

 one above the other in such a manner as to present the outline of an 

 inverted, truncated cone. 



The author, after referring to similar though less perfectly de- 

 veloped curvilinear arrangements which have been seen in lava and 

 basalt in other situations, throws out suggestions as to the cause of 

 these remarkable appearances. He objects to the opinion of Breislac, 

 that the vertical and horizontal fissures noticed by him are referrible 

 to contraction produced by the sudden cooling of a heated mass ; and 

 he adduces an instance of a lava current having flowed into the sea, 

 and been thereby subject to most rapid refrigeration, without possess- 

 ing the least fissure in its substance. The author believes that the 

 production of fissures, of prismatic forms, and of the curvilinear ar- 

 rangements, in lava and basalt, depends on uniform forces of attrac- 

 tion acting on the mass while in a fluid condition. He appeals, in 

 particular, to the spherical, el'iptic, and parabolic forms observed by 

 himself in proof of the agency of central points of attraction having 

 acted on surrounding particles, and influenced their arrangements. 



