ANNIVEKSARY ADDRESS OF THE PRESIDENT. 99 



rounded quartz-pebbles may be detected, while here and there a 

 reddish shaly band, or a layer of fine pebbly conglomerate with 

 quartz-pebbles an inch in length, shows at once the bedding and the 

 dip. Mr. W. W. Watts, who with Mr. A. McHenry of the Irish 

 Staff of the Geological Survey, accompanied me over this ground, 

 informs me that the slides which have been prepared from the 

 specimens we collected completely confirm the conclusions reached 

 from inspection of the rocks in the field. ^ He finds among the 

 angular grains slightly damaged crystals of felspar, chiefly ortho- 

 clase. Many portions of these felspathic grits reminded me much 

 of the detrital Cambrian rocks which in the Yale of Llanberis have 

 been made out of the pale felsite of that locality. 



(b). Upper Old Red Sandstone. 



In the northern half of Britain, where the Old Bed Sandstone is 

 so weU displayed, the two great divisions into which this series of 

 sedimentary deposits is divisible are separated from each other by 

 a strongly marked unconformability. The interval of time repre- 

 sented by this break must have been of long duration, for it witnessed 

 the effacement of the old water-basins, the folding, fracture, and 

 elevation of their thick sedimentary and volcanic accumulations, 

 and the removal by denudation of, in some places, several thousand 

 feet of these rocks. The Upper Old Hed Sandstone, consisting so 

 largely as it does of red sandstones and conglomerates, indicates the 

 return or persistence of geographical conditions not unlike those 

 that marked the deposition of the lower subdivision. But in one 

 important respect its history differs greatly from that which I have 

 sketched for the older part of the system. Though the Upper 

 Old Bed Sandstone is well developed from the Cheviot to the Ochil 

 Hills, and over so much of Ireland, no trace has ever been there 

 detected in it of any contemporaneously erupted volcanic rocks. 



' Mr. Watts has also examined the microscopic structure of the felsite of 

 Benaun More. He saj^s that the spherulites appear to have a micropegmatitic 

 structure, owing to the intergrowth of quartz and felspar. In some parts of 

 the rock the spherulites, from '02 to '01 inch in diameter, are surrounded 

 by exceedingly minute green needles, possibly of hornblende, while inside some 

 of them are small quartz-grains. Larger porphyritic felspars occur outside the 

 spherulites, some being of plagioclase, but most of orthoclase. The spherulitic 

 structure is not so well developed near the felspars, A few of the large Bodules 

 are hollow and lined with crystals ; while some of them show a finely con- 

 centric lamination like the successive layers of an agale. 



