510 PERLITIC AND SPHERULITIC STRUCTURES IN LAVAS. 
in rocks of modern date, confined to vitreous varieties, the inference 
was safe, when applied to ancient rocks, that they were once 
glass. 
Dr. Surrsner asked if an analysis of the rock had been made. 
If the rock was a true perlite, there should be about 80 per cent. 
of .silica ; if the rock was altered, one might expect a large excess 
of magnesia. 
Prof. Ramsay said that the character of these lava-flows was 
evident even without microscopic examination. He recapitulated 
the evidence which had persuaded him of this when surveying the 
district, and expressed doubt as to the rocks at the base of the 
Cambrian in North Wales being true lava-flows. 
Dr. Hicxs said he thought there was no reason why a perlitic 
structure should not occur in rocks of Bala age. He thought the 
first spherulitic rocks recognized in this country had come from 
rocks of Arvonian age at St. David’s. 
Mr. Baverman said that modern lava-flows often cover very large 
areas, as in North America and India; so the mere distance of the 
Wrekin from Wales would be no difficulty. 
Mr. Rurtyy doubted whether spherulitic structure was always . 
connected with vitreous. He did not see that the presence of mag- 
nesia would prove or disprove alteration. He did not think a rock 
could be vitreous if solidified at a great depth, since it would hardly 
be able to cool with sufficient rapidity. 
