228 Geological Society. 



The author next proceeds to describe the structure of tlie neigh- 

 bourhood of Oporto. The city stands upon a low ridge of granite, 

 cut through by a defile in which the Douro flows. The granite, 

 composed of quartz, felspar, mica, and hornblende, in the immediate 

 vicinity of Oporto is hard, but at a short distance from it, is decom- 

 posed even to a considerable depth beneath the surface. 



To this formation succeed, a granitic gneiss, chlorite slate, alter- 

 nate beds of anthracite and conglomerate derived from the subjacent 

 rocks, and chlorite slate again. 



An Essay " On the Curvilinear Structure of Lava," by Signer 

 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 j 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 lieated 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, elliptic, 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. 



May 2. — A paper was read, " On the Geological Structure of the 

 North-eastern Part of the County of Antrim," by James Bryce, Jun. 

 Esq. M.A. Member of the Belfast Natural History Society, &c., and 

 communicated by Roderick Impey Murchison, Esq. P.G.S. 



In this memoir the author enters into a minute description of the 

 physical features and geological constitution of a portion of the di- 



