of Leadhills, Wanlockhead, and Glendinning. 403 



only found associated with trachytes or basalts. This results from 

 observations made on both continents. Transition diorites are most 

 common on the southern aspect of the transition rocks of Dumfries- 

 shire.* 



Pitchiione. — As early as in 1805, Professor Jameson made known 

 the peculiar formation of pitchstone, which occurs in Todshaw Hill, 

 and the hills called Castle hill, Watch Craig, and Wat Carrick, on 

 Eskdale Muir. This pitchstone sometimes contains crystals of 

 feldspar, becoming a stigmite or pitchstone porphyry, and I found 

 it accompanied by a compact feldspar, or petro-siliceous rock, with 

 crystals of glimmering, and sometimes of vitreous, feldspar. This 

 rock also occurred in globular and in columnar concretions. Con- 

 temporaneous masses of pitchstone could be observed inclosed in 

 the trachytic rock, as were also distinct masses of the same rock 

 verging into pitchstone in the centre. This formation was refer- 

 red to the newest flcetz trap formation by Professor Jameson. De 

 Humboldt was also much embarrassed by the connexion, in the 

 equinoctial part of the new continent, of porphyries often argenti- 

 ferous with trachytes containing obsidian. 



Contrasting the mineralogical structure with the physical aspect, 

 we find that the central districts of this country of mountains, in- 

 cluding the culminating points of the Lowthers and Hartfell, are 

 composed of clay slate, in many places fit for economical purposes. 

 Grey wacke slate succeeds towards the sides of the chain, where com- 

 mon greywacke, which appears subordinate to the former, consti- 

 tutes rounded hills of a lesser elevation. 



To the south, the transition rocks are separated from the coal 

 formation by trap rocks, while to the north they are succeeded, 

 though at some distance, by dome-shaped mountains of phonolites, 

 as Tinto Hill, &c. 



Though we may distinguish several well-marked groups or asso- 

 ciations, from the structure and relations of composition, or the 

 oryctognostic characters, in the transition formations, yet, as De 

 Humboldt has remarked, the constancy of binary or tertiary asso- 

 ciations characterizes the transition formations, much more than 

 the analogy which the succession of homonymous rocks presents in 

 every group, and thus clay-slate and black limestone, clay-slate and 

 porphyries, clay-slate, diorites, and greywacke, porphyry and sie- 

 nite, granular limestone, and anthracitous mica-slate, are observed to 

 form geognostic associations in countries the most remote from one 

 another. This is eminently the case with the transition forma- 



* An intelligent miner of Leadhills took me to the summit of a hill in that 

 vicinity, which was literally covered with highly crystalline diorites, in con- 

 centric layers, and which he supposed were in situ, and belonged peculiarly to 

 that hill, the till or soil of which he made me remark was much deeper than 

 that of any other hills in the vicinity. Was this a formation of greenstone in 

 balls, similar to that to the south of Popayan, and at the Alto de los Robles in 

 Mexico ? Greenstone boulders are not, however, uncommon in the plains of 

 Dumfries-shire. 



