Geological Society. 293 



equal thrust. The condition of equiUbrium of any one of the be- 

 fore-mentioned elementary prisms being expressed by a diiferential 

 equation in terms of those variables, the integration of that equation 

 gives the vertical coordinate of any surface of equal thrust in terms 

 of the total thrust down to that surface and of the horizontal co- 

 ordinate. The integral obtained belongs to a class first investigated 

 by Fourier. 



An approximation to the forms of the surfaces of equal thrust 

 is obtained by a simple graphic process, first employed by Prof. 

 WiUiam Thomson in connexion with the theory of electricity. 



It is shown incidentally how the same integral may be applied to 

 determine the intrados from the extrados of any arched rib, loaded 

 only with its own weight. 



The pressure on a surface of equal thrust is vertical ; the pressure 

 on a vertical plane at a given point is parallel to the surface of equal 

 thrust traversing that point. When the upper surface of the mass 

 of earth is one plane, horizontal or inclined, the surfaces of equal 

 thrust are planes parallel to it. When the upper surface presents 

 elevations and depressions, the surfaces of equal thrust have corre- 

 sponding elevations and depressions, gradually vanishing as the 

 depth increases. 



The principles of the paper are applied to the determination of 

 the pressure of earth against walls, and the power of earth to sustain 

 buildings. The weight of the building which a horizontal bed of 

 earth will sustaiu, exceeds the weight of the earth displaced by the 

 foundation, in a ratio which is a function of the angle of repose. 



GEOLOGICAL SOCIETY. 



[Continued from p. 214.] 



February 25. 1S57.— L. Horner, Esq., V.P., in the Chair. 



The following communications were read : — 



1 . " Notice of the late Earthquake at Crete." By H. S. Ongley, 

 Esq., H.M. Consul in Crete. From the Foreign Office. 



Tiiis was communicated in three despatches relating to the occur- 

 rence of the earthquake in Crete, in October 1S5G, accompanied 

 with much destruction of property and loss of life at Canea, Retimo, 

 and the neighbouring villages. 



2. " Description of some remarkable Mineral Veins." By Prof. 

 D. T. Ansted, M.A., F.R.S., F.G.S. 



1. San Fernando Copper Lode, Cuba. — The district in which these 

 lodes are found consists of granites and syenites, jiassing into por- 

 phyritic rocks, and partly covered with calcareous conglomerate 

 and limestone. The granites are aff'ected by systems of joints which 

 heave each other, and by veins of felspar occasionally containing sil- 

 ver. Tlie mineral fields occur in calcareous porphyries passing into 

 true porphyries, and into conglomerates. The upper lodes, which 

 range E. and W., occur in the altered porphyries, and consist of two 

 groups, which have been traced about a mile in length ; the width of 



