256 Prof. Sedgwick on the Geological Structure 



marked on the geological map of Scotland), break out in 

 various parts of the chain from one end of it to the other, 

 and sometimes pass into a form of serpentine. Many dikes, 

 both felspathic and angitic, are seen in the cliffs and quarries ; 

 and we occasionally find bands of porphyry alternating with 

 the slates ; but in no instance did the author find any good 

 example of that singular combination of porphyry, trappean 

 shale (schaalstein), and slate, which gives such an impress 

 to the higher mountains of Cumberland. 



This fact explains the great difference in the features of 

 the neighbouring Scottish and Cumbrian chains, and may 

 perhaps help us in explaining the almost entire absence of 

 slaty cleavage in the former. 



The lowest fossils in the two chains appear to be grapto- 

 lites. To this fact the author's attention was often called ; 

 but he does not pretend to identify the pyritous Moffat slates 

 with the Skiddaw slates. The present evidence is not suf- 

 ficient to bear out such a conclusion ; and the whole of Moffat 

 group is only provisional. 



On the second group he, from defect of evidence, makes 

 no comparative remarks. The South Girvan group is com- 

 pared with the Bala and Coniston limestone. The North 

 Girvangroup is compared with the grits and shelly sandstones 

 which overlie the Llandeillo limestone. The evidence for these 

 conclusions will be found in the following list of fossils. 



The Balmae group is arranged provisionally as the fifth and 

 highest, on evidence formerly laid before the Geological So- 

 ciety of London ; but from physical evidence, as well as from 

 the list of fossils, which (with the kind assistance of Messrs 

 Underwood and Fleming) he derived from the shores of 

 Kirkcudbright Bay, the author would rather place this Balmae 

 group on a lower parallel, and perhaps lower than the Stincher 

 limestone. 



Lastly, the author remarks, that the Grampians had pro- 

 bably their greatest elevation after the period of the old red 

 sandstone, which is of enormous thickness and thrown off' 

 vertically from the older rocks of the chain ; but the undula- 

 tions and principal elevations of the frontier chain were pro- 

 duced before the deposit of the old red sandstone. For the 



