670 Professor Sedgwick and R. I. Muechison, Esq.^ on the 



west of Lammerton ; and at the western end of Wheal Friendship * ;) at the 

 base of the culm series, which it is extremely difficult to separate from the 

 lower rocks ; but there is no ambiguity as to the order of superposition. In 

 like manner, in North Devon, the beds of the highest group above described 

 (No. 5,) pass regularly under the base of the culmiferous rocks, without the 

 intervention of any conglomerate, or other appearance marking an interrup- 

 tion of deposit. But when one shaly mass rests on another, and is mecha- 

 nically derived from it, we need not wonder at such a mineral passage. We 

 have seen the old red sandstone on the coast of Caithness resting on gra- 

 nite and forming a most distinct mineral passage into it : and it was from 

 overvaluing such apparent passages, that Macculloch was, for many years, led 

 to regard the old red sandstone of the northern coasts of Scotland, as a part of 

 his primary system. On the whole, therefore, we consider these supposed 

 passages of little value in determining the age of the culm series, which we 

 shall endeavour to discuss on other evidence f. 



Lower Culm Measures. 



The lower part of this series is made up of dark-coloured shale ; sandstone, sometimes coarse, 

 but very rarely passing into a conglomerate ; siliceous flagstone ; and calcareous shale, here and 

 there, containing subordinate beds of black limestone. The several members of the group are 

 irregularly developed, and one part is expanded at the expense of the others ; but the calcareous 

 portion is very constant, and its position is near the top of the lower division. Where the bot- 

 tom shales rest on the earthy slates of the older groups, it is not always possible to determine the 

 exact boundary of the two formations. This difficulty, however, such as it is, very seldom occurs; 

 as the inferior group abounds in fossils, while the culm slates are without fossils, and are also 

 distinguished by their structure, being black and pyritous, and by occasional anthracitic stains 

 between the laminae. 



* The western end of Wheal Friendship, (an east and west copper vein,) near St. Mary Tavy, 

 undoubtedly runs into the lower culm-measures ; the inferior and productive sinkings are pro- 

 bably in the older slates. On the other hand, Wheal Betsey, a north and south lead vein of the 

 same mining field, is entirely in the lower culm shales and grits. We first thought an important 

 inference might be drawn from this fact ; but we afterwards found several east and west copper 

 veins, further north, in the undoubted culm-measures. 



t We forbear to describe the confused junctions near Chudleigh ; where one of the great fos- 

 sihferous limestones of South Devon is surrounded by, and might seem to be imbedded in, the 

 culmiferous rock. The phenomena are easily explained by the intervention of a fault. Neither 

 do we purpose to describe in any detail the complicated junctions near Ugbrook Park, or an out- 

 lying basin of culm-measures south of Newton Bushel. All that side of Devon is extremely dis- 

 located ; but the phenomena may be all explained, when the true order of superposition has been 

 once settled on the evidence of uninterrupted sections. 



