GEOLOGY OF DUMFRIESSHIRE. XXXI. 



grits, occasionally conglomeratic, and shales are extremly barren. Only 

 a few forms have been obtained from some of the shales, including Mono- 

 graptus exiguus, Dexolites gracilis, and Crossopodia Scotica. The Havvick 

 type of the Llandovery rocks consists of grey sandy shales, flags and 

 greywackes, associated with pale-coloured clayey shales, and red shales. 

 The shales have yielded Protovirgularia Harknessi. 



The foregoing strata are succeeded southwards by the representatives 

 of the Wenlock formation. A line drawn from the head of Ewes Water 

 in Eskdale, south-westwards by Lockerbie to Mouswald, marks the boun- 

 dary between the Llandovery and Wenlock rocks. Consisting of brown 

 crusted greywackes, flags and shales, resembling to some extent the 

 Hawick series, they are readily distinguished by certain bands of dark 

 shales yielding graptolites characteristic of the Wenlock beds of other 

 countries, as, for example, Cyrtograptus Murchisoni, Monograptus 

 vomerinus, Monograptus colonus, Monograptus priodon, etc. Occasion- 

 ally thick zones of olive-coloured shales are met with, resembling the 

 typical Wenlock shales of Wales. Along the fertile tract stretching 

 from Langholm to Ruthwell, these Upper Silurian strata are covered 

 unconformably by the Old Red Sandstone and Carboniferous rocks. 



At the northern boundary of the county in the the basin of the Span- 

 go Water, the Silurian rocks are invaded by a mass of granite, probably 

 of the same age as the granite masses of Galloway, which are later than 

 the Upper Silurian time, and older than the Upper Old Red Sandstone. 

 Excellent examples of the contact metamorphism produced by this 

 igneous intrusion are to be found in the zone of altered Silurian rocks 

 surrounding the granite. Most of the dykes of felsite, diorite, and other 

 igneous rocks traversing the Silurian area are probably of the same age. 



Towards the close of the Silurian period the marine deposits, which 

 had accumulated during that long interval of time, were elevated so as 

 to form a prominent barrier of land. In the hollows worn out of this 

 ancient table-land the strata belonging to the Old Red Sandstone, 

 Carboniferous, Permian, and Triassic periods, were deposited. These 

 newer formations have, however, been so much denuded that only 

 isolated fragments remain of what were once more extensive deposits. 



The representatives of the Old Red Sandstone belong to the lower 

 and upper divisions of that system. Along the County boundary in 

 Upper Nithsdale the members of the lower division occupy a limited 

 area, stretching north-eastwards from the northern margin of the San- 

 quhar coal field. They consist of sandstones and conglomerates, 

 prominently developed on the slopes of Corsoncone beyond the County 

 boundary, where they are associated with contemporaneous volcanic 

 rocks. They form part of the great belt of lower old red strata, 

 stretching from the Braid Hills, near Edinburgh, into Ayrshire. 

 The upper Old Red Sandstone, on the other hand, forms a narrow 

 fringe underlying the carboniferous rocks from the County boundary 



