1855.] MURCHISON LESMAHAGO SILURIANS. 21 



two localities compared are a few miles only distant from each other, 

 the geologist must see, by the facts laid before him, how very rapidly 

 mineral matter of one sort thins out and is represented at a short 

 distance only by a very different stratum. 



When the trigonometrical surveys of these important mining tracts 

 shall have been published, the geological surveyors will determine the 

 extent to which the coal-fields of Scotland can be distinguished as 

 consisting of lower and upper masses, a subject already treated of by 

 Mr. Page *, and will explain with precision whether to the south of 

 Edinburgh there are or are not strata of younger age than those now 

 alluded to. In the mean time it is clear, that all the coal-tracts around 

 Lesmahago belong to the older or Mountain Limestone series. They 

 are, in short, of the same age as the coal-fields of North Northumber- 

 land, Berwickshire, and other tracts in Scotland; and in foreign coun- 

 tries, as those of the Donetz in Southern Russia, and of Kosloo in Asia 

 Minor, both of which are subordinate to bands of Productus-limestone. 



Igneous Rocks of the District of Lesmahago. — Allusion has already 

 been made to certain porphyries, some of which alternate with bands 

 of the Old Red Sandstone and conglomerate, and others of which seem 

 to have been erupted through the Upper Silurian rocks and the Old Red 

 also. According to the map of Mr. Shmon, these porphyries, of which 

 there are two varieties, felspathic and quartziferous, are both chiefly as- 

 sociated with the Old Red Sandstone, and never occur in the coal-fields. 



One of the largest bands of porphyry traverses the River Clyde 

 below the Fall of Stonebyres, and, trending to the S.W. and S. through 

 the Old Red Sandstone, sends off several branches, three or four of 

 which curve round and cross the Nethan River. 



Another branch runs to Dunduf, from whence it ranges to Todlaw. 

 This last-mentioned dome was probably a great centre of eruption, 

 from whence a long course extends from Ach Robert to the flank of 

 Bremerside Hill. 



In short, these porphyries seem to have been the active agents, 

 which, at one period alternating with the red sediments, afterwards 

 burst through the Old Red Sandstone and raised it into those dome- 

 shaped masses which separate the great coal-fields of the Clyde from 

 the detached coal-basin of Douglas. Tinto Hill and its south-western 

 ramifications constitute another and much more extensive outburst 

 of similar porphyry, which forms the south-western portion of that 

 long range of igneous rocks which extends on the E.N.E. to the 

 Pentland Hills. 



But independently of these porphyries, the parish of Lesmahago is 

 distinguished by a remarkable dyke of greenstone, which Mr. Slimon 

 has traced from the coal-field of Douglas, across the Old Red Sand- 

 stone of the Hawkshaw Hills, and then through the porphyry zone 

 of Todlaw and the Old Red Conglomerate, and which he has further 

 followed for several miles across the heath-covered Silurian hills 

 in nearly a rectilinear course. Where Prof. Ramsay and myself 

 examined this dyke, i. e. high up the Nethan Water, we found it to 

 be a fine-grained greenstone of about 25 paces in width ; having the 

 direction of 33° S. of E. The Silurian schists on either side of it 



* See Report British Assoc. Advancement of Science, 1854, Sect. p. 92. 



