jgl5 1 Geological Socicly. 149 



Icldal an-agonUe (flos ferri), pearl spar, crystallized and sandy fluor, 



witherite, and heavy spar. • , , • n i .1 r-i Vi- 



Tl>e N W. of Northumberland is occupied chiefly by the Cheviot 



Hlll« which consist of various porphyritic and amygdaloidal rocks, 



accompanied on the side adjacent to the basset of the lead-mifle 



measures by red sand-stone and grey-wacke-slate. 



A communication from W. H. Pepys, Esq. Treas. G. S. was 



"^ This paper contains experiments to ascertain the composition of 

 the sand tubes found at Drigg, from which k appears that the 

 vitrified sand of which they are composed consists ot 93 silex, 4-o 

 alumina and oxide of iron, 1 lime, 0-2 alkali. .,, xt ,, 



Nov. 4.— A paper by Dr. Berger, on the whin dykes of the North 



of Ireland, was read. , r 1 * n +1,^ 



The dykes of the district here mentioned are found at all the 

 elevations from the level of the sea to the summit of Anagh, the 

 highest mountain of Donnegal, being a range of 2220 feet. Where 

 one occurs others are generally to be found in the immediate 

 vicinitv, and all of them running in parallel, or nearly parallel, 

 directions to each other. This parallelism moreover obtains not 

 only in adjacent dykes, but in almost all of this district, their 

 general direction being N. W. and S. E., or perpendicular to that 

 of the strata and the metallic veins, both of which are mtersected 



^The"!vidth of dykes varies considerably, from a few inches to 

 several hundred feet ; and upon the whole it appears that those 

 which intersect primitive strata are narrower than those which 

 intersect secondary strata. The substances which compose dykes 

 bein<r for the most part In a state of greater induration than the 

 beds which they traverse, they may be perceived protruding above 

 the general surface, either in the shape of vertical walls, m some 

 instances 40 feet high, or of a mound composed of loose stones. 

 The depth to which the dykes descend is wholly unknown ; nor do 

 they appear to become narrower in proportion as they descend. 



The usual materials of dykes are trap and green-stone, with 

 lydian-stone, flinty-slate, grey-stone, and wacke. l^errugmous 

 sand-stone also occurs, but more rarely. The structure ot a dyke 



presents columnar, or ovoid, or nearly rectangular concretions, 

 laterally aggregated, and with their axis perpendicular to the walls; 

 and the more compact the trap is the more apt it is to assume the 



columnar form. . . , , 



jVoy 18— The Secretary reported that a communication had 



been received from Wm. Phillips, Esq. M.G.S. on the granite ot 



Cornwall. 1 .• /- 



A notice of some fossils found in the neighbourhood "^ > =""- 

 bridge, by the Rev. 1. Hailstone, VVoodwardian Professor, M.G.b. 



' The dialk hilU of Cambridgeshire, where they terminate in the 



