72 " Proceedings of Philosophical Societies. [Sas. 



Agnes and Cligga Point, traversing or lying on the surface of rocks 

 of tortuous killas, or clay-slate. The veins themselves vary in thick- 

 ness from forty feet to half an inch. Their general character is por- 

 phyritic, consisting of a ba^c composed of minutely aggregated 

 quartz, mica, talcite, and probably felspar, in which are imbedded 

 grains and crystals of quartz, felspar, chlorite, mica and talcite in 

 small patches. Sometimes the porphyritic character is superseded 

 by a more completely crystalline one approaching to granite, and 

 containing small veins of tin stone. Sometimes again the veins 

 consist of quartz and tourmalines, forming a rock very nearly 

 resembling that of St. Roche. 



The clav-slate adjacent to the veins is more crystalline than else- 

 where, and sometimes is scarcely to be distinguished from gneiss. 

 Mr. Conybeare considers the veins and the rock in which they occur 

 to be of contemporaneous origin. 



A paper was also read entitlecf " A Description of some vSpeci- 

 mens from the neighbourhood of Cambridge, by Henry Warbur- 

 ton, E^q M. G. S. 



These specimens formed part of a bed of rubble covering the 

 summit of a^iillockcf grey, or the lower chalk, about five miles S. VV. 

 of Cambridge. 'J his hillock, like several others in the same county, 

 is situated to the west of the great range of chalk, being surrounded 

 by the blue marl or gault, as it is provincially termed, from which 

 the overlying bed of chalk is separated by a thin bed of green sand. 

 The rubble, besides consisting of chalk and flint, also contains 

 shell lime-stone, angular pieces of green-stone, and certain organic 

 remains belonging to older beds than the chalk; but as all these 

 beds i -asset more or less to the west of the place where these frag- 

 ments are now to be found, the circumstance is considered by Mr. 

 Warburton as indicating an ancient current, the course of which 

 ■was from west to east. 



A paper was also read entitled " Observations on Glen Tilt," 

 byDr M'Culloch, V. P.G.S. 



That part of Glen Till which is the subject of the present paper, 

 extends four or five miles from Forest Lodge to Gow's Bridge. It 

 consists of primitive -slate, assuming the form of clay-slate, of mica- 

 slate, and of hornblende-slate, with which are interstratified various 

 beds of gianular lime-stone, more or less micaceous. Near Gow's 

 Bridge the stratification is perfectly regular and uninterrupted, but 

 higher up towards the Lodge it is tra\'ersed by granite rock and an 

 infinite multitude of granite veins of various sizes. Where this 

 latter roi !< makes its appearance, the even course of the schistus is 

 interrupted in proportion to the magnitude of the mass of granite. 

 When the granite, slate, and lime-stone are in contact, the latter ig 

 highly indurated and penetrated by «ilicious matter. 



