GEOLOGICAL SOCIETY OF DUBLIN. 105 



Its thickness varies from 20 to 60 or 80 feet in different places. 2. Next 

 is a series of beds of red sandstones and red shales, from 200 to 600 feet 

 thick. 3. The upper part exhibits thick beds of sandstone, of a whitish 

 or yellowish colour. This upper part is the yellow sandstone, which 

 sometimes contains a band or two near the top of black shale, interstra- 

 tified with thin beds of limestone, full of the fossils of the limestone. 

 This yellow part of the Old Eed varies from 50 to 200 feet in thickness. 

 The whole thickness of the Old Red Sandstone in Ireland averages about 

 1000 feet. 



I am aware that this subdivision, which, in former times, was in- 

 cluded in the Carboniferous formation, has been recently cut away from 

 it, and joined with another rock, which lies below it, sometimes directly 

 in contact with it, and both together now called Old Red Sandstone. I 

 could wish this separation had never been made, for it blots out a great 

 line of demarcation which nature has made, and which ought not to 

 have been blotted out. Botanists and zoologists, in devising their sub- 

 divisions, seek for the strongest lines of demarcation between the groups 

 of their several orders and classes. This golden rule seems to have been 

 totally overlooked in our science. The strongest and most prominent 

 boundary line made by nature in Geology is a sedimentary unconfor- 

 mability. It is the chasm between two formations. It marks the time 

 of a change, sometimes of a great catastrophe, which occurred at the end 

 of one formation, and before the commencement of depositing materials 

 for another. At the end of the Silurian period, and immediately before 

 the Carboniferous formation, there appears to have been a time of un- 

 usual convulsive movement in the system of rocks which had then been 

 formed. By this movement the beds were made into great, folds, the 

 tops of those folds often broken and carried away, leaving those beds 

 turned up on their edges. This period of time coincides with one of the 

 chasms I have been describing, in which no rock appears to have been 

 deposited. 



The conglomerate of the Old Red Sandstone, which forms the base 

 of the Carboniferous system, was the first or lowest deposit laid down 

 on the older beds after the period of disturbance just described. It is 

 spread out, in all places I know, upon the upturned edges of the sup- 

 porting rocks, in beds varying but little from the horizontal — thus 

 forming the foundation of a new system. Those supporting rocks are 

 different in different places : sometimes mica slate, more generally clay 

 slate, or gray grit, sometimes brownstone, sometimes quartz rock, or 

 porphyry, or greenstone, or granite. I look upon this conglomerate as 

 a most important index in geology. Besides being the beginning of 

 a new system of rocks, it is the boundary between two distinct periods 

 of organic life ; the fossils below it differ in genera and species from 

 those above, and, besides this, there is a well-marked difference in the 

 lithological character of the rocks also, the lower and older being much 

 harder and more quartzose ; the upper, softer. 



The fossil evidence, so far as it goes, supports this view, and forms 

 a link to tie this Old Red Sandstone inseparably into the Carboniferous 



VOL. IV. p 



