1815.] Geological Society. 145 



part covered by the magnesian lime-stone. It is in the form of a 

 trough, the extreme length of which from N. to S. is 58 miles; and 

 its breadth from the sea-coast westwards is about 21 miles. The 

 general inclination of the coal strata is one yard in 20, but subject 

 to considerable local irregularities. The earthy beds which separate 

 the coal seams from each other are potter's clay, slaty-clay in 

 various states of induration, and sand-stone both massive and slaty. 

 Of the latter beds some are quarried for flag-stone ; and one, a buff- 

 coloured fine-grained sand-stone, called the grind-stone sill, fur- 

 nishes the celebrated Newcastle grind-stones. Thin beds and 

 nodules of clay-iron-stone, the latter containing impressions of 

 ferns and bivalve shells, occur in the shale or slatj'-clay. A few 

 dykes of basalt or green-stone intersect the coal fomiations. The 

 best iinown is called Walker's Dyke. It is composed in Walker 

 Colliery of two solid and parallel walls of green-stone of the thick- 

 ness of three yards and six yards, vvith an interval of about 1 1 feet, 

 composed of fragments of green-stone and sand-stone imbedded in 

 blue slate. The dyke is perfectly vertical, and divides, but does 

 not dislocate or heave the strata which it traverses. The coal on 

 each side of it, to a distance of three to six yards, is converted into 

 a hard cellular cinder, the cells of which are often occupied by 

 calcareous spar and sulphur. 



Fissures or slips are of frequent occurrence in this as in every 

 other coal-field, and occasion much trouble and expense to the 

 miner. Numerous mineral springs, containing moie or less of 

 common salt, occur in the whole of the coal-field. 



^Ipril la. — A communication from S. Solly, Esq. on the newer 

 formations, particularly that of floctz trap, was read. 



From the remarkable differences, notwithstanding their general 

 resemblunce, which prevail amongst the floctz t:ap, and other recent 

 floetz rocks of the same species in different countries, Mr. Solly is 

 induced to suppose that these are not parts of an universal formation, 

 as is held by Werner and his pupils, but that they originate from 

 local deposites. 'J'hese local depositts are considered l)y the author of 

 the paper as owing their peculiar structure and other characters 

 |mrily to the influence of heat (Icrivid from the electric fluid, and 

 partly to the aciion of crystalline polarity operating within them, 

 while ihcy were apparently in a quiescent state. 



A paper by Dr. Berger on the geology of tlie North of Ireland 

 was begun. 



lilvy (j. — The reading of Dr. Berger's "pt^ex on the geology of 

 the North of Ireliujd was continued. 



A paper eutlilt-d A l)escripti( n of the Tunnel of Tavistock Canal 

 tlirough Morwcl Down, in the County of Devon, by John Taylor, 

 Esq. M.G.S. was read, 



Morwcl Dov.n is a hill near Tavistock, and to thu west of that 



place, which i-c|)arates the valley of Tamar from that of the Tavy. 



Ihe hiight of this hill is about 700 feet al;ove the tide-way in the 



liver 'I'ainar. It is composed of jchist (in the dialect of the country 



Vol. V. N'^ Ji. K 



