8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1 /, 



of this transverse system. They are shallow bays, nowhere exceeding 

 seven fathoms in depth, separated by an isthmus about nine miles 

 broad of low land, consisting entirely of boulder clay, loose sands, 

 and peat bogs. The whole country, as we proceed from the highest 

 mountains of Kirkcudbrightshire towards the west, becomes less and 

 less elevated above the sea ; so that an amount of depression, which 

 in the more eastern part of the chain would form a valley with a river 

 flowing through it, becomes an arm of the sea as we advance to the 

 west. 



That this transverse system of valleys or fissures is of high geo- 

 logical antiquity, seems indicated by the fact, that the most western 

 of these depressions, the bay of Loch Ryan, is partly occupied by 

 unconformable strata of clay, sandstone, and conglomerate about 400 

 feet thick, the lowest bed of which contains Stigmaria ficoides and 

 Calamites. A fracture therefore had occurred across the grej^wacke 

 range, and a valley had been scooped out in it prior to the deposition 

 of these secondary rocks : and as this bay appears to be one of a 

 system, it would follow that all these parallel fissures took place pre- 

 vious to the deposition of the coal-measures. 



The rocks consist of the usual varieties of coarse and thin-bedded 

 greywacke, and clay-slate in which true slaty cleavage can never be 

 detected, with occasional interv^ening beds of plutouic rock, which in 

 Wigtownshire is usually felspathic, varying from a nearly pure felspar 

 rock to a syenite. 



These plutonic rocks appear to follow the direction of the sediment- 

 ary beds, and to be interstratified with them ; yet I am persuaded they 

 are most frequently, if not always, of an intrusive character. The great 

 extent of coast in Wigtownshire, washed by a boisterous sea, affords 

 favourable opportunities for observing : near the Corswall Lighthouse, 

 for example, an extent of many acres is washed bare by the Irish Sea. 

 In such situations (and they are numerous), whenever a bed of plu- 

 tonic rock is traced far, it wdll be found to cut transversely across the 

 greywacke beds, which are thus seen to abut against it. The plu- 

 tonic rock generally resumes its direction between the beds ; so that 

 if the point where it traverses them were not exposed, it might lead 

 to false conclusions. The sedimentary beds are usually altered near 

 their contact ; they become more or less porphyritic, quartz veins are 

 frequent in the \'icinity of the trap, dark shales become white and 

 sometimes red ; and these effects are seen on both sides, and to the 

 same distance from the dyke. 



I shall now describe briefly the section exhibited along the Irish 

 Channel from the Mull of Galloway to Corswall Point. 



At the Mull of Galloway and for some distance to the north, the 

 beds are nearly vertical. From the hill called Dunman to a point 

 half-way between Portencorkrie Bay and Clanyard Bay, the coast for 

 about two miles consists of granite, which covers a rectangular sur- 

 face, extending about two miles inland to the eastward : it is a 

 quadruple granite of quartz, felspar, mica, and hornblende, sometimes 

 pink, but principally grey ; it is distinctly columnar, dividing into 

 prisms about a foot square, and is too subject to decomposition to be 



