E. MINERALOGY AND GEOLOGY. 227 



slimy in the fingers, but if the animal matter be washed 

 away it is found quite hard. Chemically the cliief constitu- 

 ent of the whole is carbonate of lime, in the proportion of 

 ninety-five to ninety-eight parts in one hundred, with one 

 and a half to four parts of organic matter, and some earthy 

 ingredients, such as phosphate of lime, with a trace of silica, 

 amounting usually to less than one per cent. Forchhammer 

 found 2.1 per cent, of magnesia in Coralliumrubrum,and 6.36 

 in Isishippuris. The sources of these constituents are the 

 sea-water and the ordinary food of the polyps, the process 

 of absorption, assimilation, and secretion going on in them 

 as in all animal organizations. Engineer^ XL., 1034, 281. 



VOLCANIC DUST. 



A microscopic examination of the dust which fell in parts 

 of Sweden and Norway on the night of March 29-30, 

 1871, has led M. Daubree to believe that it proceeded from 

 a volcanic eruption in Iceland. The dust was found to be 

 composed of fragmentary transparent grains, some colorless, 

 others more or less brownish-yellow. Minute crystals of 

 feldspar and pyroxene were recognized. Academy^ May 8, 

 1876. 



VOLCANIC HISTORY OF IRELAND. 



In the annual address delivered before the Royal Geolog- 

 ical Society of Dublin, Professor Hull has given an interest- 

 ing account of the volcanic history of Ireland. He shows 

 that in the Lower Silurian there are numerous examples of 

 volcanic rocks in several parts of the island ; thus in the 

 counties of Wicklow and Waterford there are numerous 

 sheets of felstones and porphyries accompanied by beds of 

 ash and volcanic breccia, and similar conditions in other dis- 

 tricts. All of these volcanic rocks are to be rcG:arded as 

 having been erupted from vents sporadically breaking out 

 over the sea-bed of the period, they having been in turn 

 covered up by fresh sediments. In the Upper Silurian there 

 is little evidence of volcanic action ; in the Devonian there 

 are more, and in the Carboniferous they are still more de- 

 cided. Coming down in the geological history, it is neces- 

 sary to pass over to the Tertiary before the products of the 

 volcanic fires are again met with. In tliis period they were 



