On the Metamorphic Rocks of Counties Sligo and Leitrim. 361 



times nearly one incli in diameter at the base. They are enclosed 

 in milk-white quartz, which retains a perfect cast of the form 

 and striation of the crystal, and must, therefore, have been depo- 

 sited later. The contrast presents a very beautiful appearance. 

 Associated with these is pink felspar in very large crystals. 



Olivine is disseminated in small crystals in many places. 



Garnets are found near Rosses Point, and in the coarse gneiss 

 west of Carrigeencor Lake. They are very small. 



Copper Minerals. — Chalcopyrite occurs in quartzite near Shana- 

 van's Bridge, in an old copper working. 



Azurite. — Very beautiful specimens were obtained at the same 

 locality. 



Malachite is found at the same locality associated with the 

 other -copper minerals. It also occurs in a decomposed vein 

 in quartzose schist, associated with galena, one mile N.E, of 

 Manorhamilton, 



Qalena in small quantity in last two localities. 



Cobalt. — In the last locality some specimens, apparently con- 

 taining cobalt, were obtained. 



Pyrites. — This is found in small specks throughout. It is 

 associated with the copper minerals at the mine at Shanavan's 

 Bridge. 



Geological Age of the Metamoephic Rocks. 



On this point we have as yet no definite information. That 

 they are extremely old we cannot but admit. Unfortunately, in 

 the Sligo district the links between the carboniferous and older 

 rocks are wanting, and it is only possible to judge from analogy 

 and resemblance to other rocks of known age. Accordingh^, they 



have been ascribed to a very early age by most geologists 



amongst others, Mr. Kinahan places them in the Cambro-Silurian * 

 while Professor Hull traces their origin still further back, to the 

 Laurentian system t ; and there is indeed no reason to deny that 

 they may even be of that vast age. 



The Section wiU demonstrate the difficulty of determinino- their 

 age on stratigraphical grounds alone. 



* Geology of Ireland, p. 21G. 



■i- " On the Lanrentian Eocks of Donegal, &c." Scien. Trans. Eoy. Dub. Soc. Vol I 

 Ser. 11., p. 252. ' ' '' 



