Professor Geihie on the " Fitchstone " of Eskdale. 241 



further it continues towards the south-east I have not yet 

 ascertained. 



Tln'oughout this long course the breadth of the dyke 

 varies considerably, averaging probably about forty feet. 

 Where the edge of the dyke permits observations to be 

 made, the basalt always appears to fill a fissure, either 

 vertical or inclined at a high angle with the horizon. 

 For most of its course the dyke runs through steeply- 

 dipping, folded, and fractured Lower Silurian rocks; but 

 the fissure up which it has risen has been opened in 

 these rocks as a long persistent dislocation, that has been 

 little, if at all, affected by the structure of the rocks through 

 which it runs. In Eskdale the dyke enters the Upper 

 Silurian belt, likewise without deflection, though it is ob- 

 served to curve round with the trend of the valley of the 

 Esk. Beyond Langholm it crosses the volcanic zone at the 

 base of the Carboniferous Series, and then the overlying 

 sandstone, shale, and limestone groups, ^^ith equal indifference 

 and continuity. 



Part III. — Peteographical. 



Throughout most of its course this dyke consists of an 

 ordinary crystalline-granular dolerite, weathering into the 

 usual rudely spheroidal blocks which, scattered over the 

 hills along its route, indicate its position. In Eskdale it 

 presents a more complex structure. It there consists of 

 three portions (PL V., fig. 1). On either side lies a zone of 

 the usual dolerite, about eight feet broad. Between these 

 two marginal zones comes a central band, sixteen to eighteen 

 feet broad, of a very compact, vitreous basalt — Jameson's 

 " pitchstone." In the two outer belts of rock there is no 

 feature of special importance to be noted. Along their 

 junction with the highly inclined grey^vacke and shale, 

 they assume the normal fine-grained texture, but show no 

 trace of a glassy character. The central vitreous band, on 

 the other hand, presents several curious features which 

 are readily visible in the field. The line of demarcation 

 between this band and the marginal dolerite zones is quite 



