ANNIVERSARY ADDRESS OF THE PRESIDENT. 77 



limits almost every stage may be met with, the proportion of chlorite 

 or honiblciule and of granular or pebbly quartz varying continually, 

 not only vertically, but even in the extension of the same bed. The 

 quartz-pebbles are sometimes opalescent, and occasionally larger than 

 peas. An average specimen from one of the zones of " green schists" 

 is found, on closer examination, to be a thoroughly schistose rock, 

 composed of a matrix of granular quartz, through which acicular 

 hornblende and biotite crystals, or actinolite and chlorite, are ranged 

 along the planes of foliation. 



That these rocks are essentially of detrital origin admits of no 

 doubt. They differ, however, from the other sedimentary members 

 of the Dah-adian series in the persistence and abundance of the 

 magnesian silicates diffused through them. The idea which 

 they suggested to my mind some years ago was that the green 

 colouring-matter represented fine basic volcanic dust, which had 

 been showered out during the accumulation of ordinary quartzose, 

 argillaceous, and calcareous sediments, and that, under the influence 

 of the metamori)hism which has so greatly aflPected all the rocks of 

 the region, the original pyroxenes had suffered the usual conversion 

 into hornblendes, chlorites, and mica. This view has occurred also 

 to my colleagues on the Survey, and is now generally adopted by 

 them. 



IN'ot only are these " green schists " traceable all through the 

 central Highlands ; rocks of similar character, and not improbably on 

 the same horizons, cross Argyllshire, reappear in the north-west of 

 Ireland, and run thence south-westward as far as this series of rocks 

 extends. If we are justified in regarding them as metamorphosed 

 tuffs and ashy sediments, they mark a widespread and long- 

 continued volcanic period during the time when the later half of 

 the Dalradian series was deposited. 



A remarkable confirmation of the inference that the deposition of 

 that series was accompanied by widespread and long-continued 

 volcanic activity has lately been obtained in County Tyrone. Mr. 

 N^olan, of the Geological Survey of Ireland, has described a group of 

 green basic rocks from the north of Pomeroy, containing talcose 

 schists and schist-breccias, sometimes presenting " all the appearance 

 of metamorphosed conglomerates," and including hornblendic and 

 pyroxenic masses *. On visiting that district with my colleagues in 

 the Survey, Mr. B. N. Peach and Mr. A. McHenry, 1 found that it 

 includes an undoubted core of Archaean gneiss, resembling in all 



* Geol. Maff. for 1879. 



