IxXTiii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



cleavage dips produces the fan-shaped stratification noticed in that 

 range of mountains. His explanation of the cause of cleavage is that 

 every plicated mass of matter, after flexure, consisted of hotter and 

 colder planes, so that an agency was exerted analogous to that of a 

 thermo-electric pile, inducing in the surrounding unsolidified materials 

 the special and symmetrical polarities of the particles, which have 

 been supposed the proximate cause of cleavage. 



In his paper on the Silurian rocks near Oporto Mr. Sharpe men- 

 tions that the cleavage and foliation of the gneiss, mica schists and 

 Silurian accumulations of that district would appear to form an irre- 

 gular arch over the Oporto granite, of which the diameter, if it could 

 be fully seen, would be about twenty-five miles. The perpendicular 

 cleavage observed about two miles east of Vallongo, in the middle of 

 clay slates, forms its limit in that direction, and the commencement 

 of another arch, extending to the N.E., and in which the igneous rocks 

 to the southward of Baltar would occur. The strike of the cleavage 

 would appear to be N.W. and S.E. 



In the address of last year we had occasion to call your attention 

 to the case adduced by ^Ir. Charlesworth of the preservation of the 

 soft parts of a Trigonia by means of silica, and to remind you of the 

 observations of Dr. Mantell respecting the preservation of the re- 

 mains of the soft parts of molluscs by the same substance. During 

 the past year Mr. Bowerbank brought before us a paper on a sili- 

 ceous zoophyte, Alcyonites parasiticiim. He describes a small agate, 

 the locality of which is not known, as containing a body which ap- 

 pears to have been of a fleshy texture and semitransparent, like that 

 of Alcyonidium gelatinosum of our coasts, encrusting the fibres of 

 a species of Verongia, the tubular fibres of the sponge being in 

 many places beautifully preserved. The surface of the polypidom is 

 stated to present a strongly mammillated or tuberculated appearance, 

 which jNIr. Bowerbank thinks may possibly have resulted from the 

 exhaustion of the animal, anterior to death, having prevented its 

 complete withdrawal within the polypidom. Our colleague then 

 refers to the rapid deposit of silica which could thus preserve animal 

 tissue before it was decomposed, and considers that after the first 

 quick deposit of the siliceous matter, the filling up of the interstices 

 of the tissue proceeded more slowly. Mr. Bowerbank afterwards 

 discusses the mode in which silica has been deposited. Having ex- 

 amined microscopically siliceous deposits from the Geysers, brought 

 to this country by Mr. Babiugdon, he does not find the parts arranged 

 in a fibrous crystalline manner as in chalcedony or agates, but more 

 like a mass of melted glass, not having had the conditions for crystal- 

 lization afforded. In the fossil noticed he also found no appearance 

 of crystallized arrangement of the silica, and concludes, from what 

 he has observed during the crystallization of certain salts beneath 

 the microscope, that even in the case of chalcedony the ciystallization 

 of silica may be achieved in much less time than is commonly 

 imagined. 



Respecting the production of artificial quartz and siliceous coatings, 

 Mr. Bowerbank quotes a communication of Mr. Warren de la Rue, 



