89 
1907-8.] Classification of Igneous Rocks. 
being placed within brackets (see page 151). The specific gravity of the 
vitreous rocks averaged 2 ’44 and that of the non- vitreous ones 2 ’80. 
Taking all the rocks together, 362 in number, the grand average amounted 
to 2'76, and we have here again a coincidence. The mean specific gravity 
of 736 British rocks was found by Harker to be also equal to 2‘76 (see 
Geological Magazine, 1899). 
[Tables. 
