552 Murchison's Silurian System. 



of the widely expanded series of old red sandstone, there 

 should apparently have been a total cessation of the erup- 

 tions, for the dykes just described must have been intruded 

 after the old red strata were deposited. 



The old red sandstone contains no metallic veins of any 

 value. Throughout the whole extent of the system, Mr. Mur- 

 chison found but two instances of metalliferous veins, which 

 have been deemed worthy of the slightest attention; both 

 were copper ore ; nor is there any coal in the old red sand- 

 stone, or any vestige of the vegetable matter out of which 

 coal has been formed ; an exception however is mentioned 

 in favour of one place on the banks of the Tweed, in which 

 coal seams do occur beneath the mountain limestone in the 

 upper member of the old red system. 



Mr. Murchison next describes isolated patches of old red 

 sandstone, under the term outliers. The most extensive 

 outlier composes a district of nearly one hundred miles in 

 extent, of which the forest of Cleen forms the principal part. 

 The soil of this tract is red, and numerous natural sections 

 of the rocks beneath it show they belong to the lower beds 

 of the old red system, or tilestone, resting on Silurian rocks, 

 which support the sandstone at low angles of inclination. 

 The transition or passage between the Silurian and old red 

 systems is here marked, as in other places, by hard, greenish, 

 and reddish, highly micaceous sandstone, which contain Sep- 

 toena lota and Terebratula nucula, together with casts of 

 several shells identical with those found in the tilestone and 

 upper Silurian beds. The casts of these fossils, though 

 fragile, are beautifully preserved, and the cavities in which 

 they are situated generally contain a black powder. Three 

 other outliers are described in various parts of Radnorshire, 

 but the largest of them is not above five miles in extent. 

 They show the relations of the old red system to the Silurian 

 rock, and confirm the accuracy of the sections of the former, 

 previously described. 



