Section of' Geology and Geography. 48 1 



thick. Mr. Griffith, confining himself on this occasion to the sedi- 

 mentary rocks, commenced his illustrations by those of a more 

 crystalline character, such as gneiss, mica slate, &c. ; and stated 

 that he considered the great groups of Ireland as corresponding to 

 those of Scotland, particularly the Northern to the Grampians, and 

 the Mourne to the Dumfriesshire mountains. The general direc- 

 tion of stratification is N.E. and S.W., though in Tyrone it is more 

 nearly N. and S., being referred to a local axis ; and in the south 

 nearly E. and W. The beds of primary limestone, associated with 

 the primary schists, are not continuous, though they occur in lines : 

 when intersected by trap dykes, they become dolomitic. The quartz 

 rock, which is also associated with these schists, is sometimes very 

 remarkable. At Dunmore Head it has the structure of orbicular 

 granite, or of some varieties of trap, for which it is often mistaken. 

 Mica slate is unequally distributed : it is abundant in the north and 

 west, less general in the south, and deficient in the Mourne or Down 

 district. Mica also, as a mineral, is not general, being in the Mourne 

 mountains often replaced by hornblende. Proceeding to the trans- 

 ition schists, Mr. Griffith stated his conviction that they would re- 

 quire subdivision, whenever materials had been collected for the 

 purpose, in the same manner as those of Wales had been divided by 

 Mr. Murchison. For example, in the older schists, neither conglo- 

 merates nor organic remains are found. In the newer greywackes, 

 the slates alternate with sandstone; and again, in the still newer 

 strata, limestone, containing fossils, alternates with the upper por- 

 tion of the schists. The old red sandstone is also considered by 

 Mr. Griffith divisible into two or three subsections, the upper alter- 

 nating with the mountain limestone. Mr. Griffith then described the 

 several coal-fields of Ireland, pointing out the distinction between 

 those of the north and south, bituminous coal being confined to the 

 northern collieries. The more recent sedimentary rocks were then 

 briefly described, more especially the new red sandstone, which un- 

 derlies the lias and chalk on the S. and E. of Antrim, and is also 

 found in Monaghan, and may be traced thence through Tyrone and 

 Derry to Lough Foyle, and round Lough Foyle to Donegal. 



Having previously described the sedimentary, he now entered on a 

 description of the crystalline rocks considered as rocks of intrusion. 

 In the Wicklow range, extending to Brandon, the granite contains no 

 hornblende, and, as previously noticed by Mr. Weaver, occurs some- 

 times as beds in mica slate. In the Mourne or Down range, the gra- 

 nite contains hornblende, which frequently predominates over the 

 mica. In Wicklow, mica slate, passing into gneiss and clay slate, 

 abuts without disturbance against the granite. In Down mica slate is 

 wanting, and the other schistose rocks are frequently disturbed. In 

 western Donegal mica slate and quartz rock are abundant, the 

 quartz rock being developed to a great extent ; and in Galway also, 

 associated with mica slate, quartz rock is extensively diffused. In 

 both these counties granite occurs, and the crystalline stratified- 

 rocks are referred to as affording distinctive characteristics of its 

 several localities. The phaenomena usually exhibited by granite 



Third Series. Vol. 7. No. 42. Dec. 1835. 3 Q 



