Lower Silurian Rocks, Scotland. 83 



and afterwards that the exposed portion should be irre- 

 gularly cut away and destroyed by processes of long-con- 

 tinued waste and decay, partly sub-aerial and partly 

 marine. 



The remaining areas in Great Britain occupied by 

 Lower Silurian rocks lie in Scotland. The southern 

 district extends from St. Abbs Head on the east to Port- 

 patrick on the west coa?t, forming the uplands of the 

 Lammermuir, Moorfoot, and Carrick Hills, fig. 55, 

 p. 287. They chiefly consist of thick banded strata of 

 grits and slaty beds, much contorted, and in the 

 western part of the area, where bosses of granite and 

 other igneous rocks occur, they are often metamorphosed. 

 The fossils which they contain prove them to belong 

 to the Llandeilo, and Bala or Caradoc series. 



In Wigtonshire great blocks of gneiss, granite, &c. 

 are imbedded in the dark slaty strata near Corswall, 

 and similar large blocks occur in Carrick in Ayrshire. 

 Where they came from I cannot say, for all the nearest 

 granite bosses in Kirkcudbrightshire and Arran are of 

 later date than the strata amid which these erratic 

 blocks are found. I therefore incline to the opinion 

 that they must have been derived from some Lauren tian 

 region, of which parts of the mainland and of the Outer 

 Hebrides then formed portions, and when I first saw 

 them I could, and still can conceive of no agent capable 

 of transporting such large blocks, and dropping them 

 into the graptolite-bearing mud, save that of icebergs. 

 One of the blocks measured by me near Corswall, in 1865, 

 is 9 feet in length, and they are of all sizes, from an inch 

 or two up to several feet in diameter. Many persons 

 have considered, and will still consider, this hypothesis 

 of their origin to be overbold, but I do not shrink from 

 repeating it, and I may mention that the same view 



o 2 



