VOL. XV.(1) | THE PRESIDENT’S ADDRESS 15 
tuffs ejected from the Precambrian craters are identical, 
both chemically and lithologically, with the products of 
modern eruptions. There are the same variations in 
composition, from the basic, in which lime, iron-oxide, and 
magnesia predominate, to the acidic, which yield a high 
percentage of silica, and in which the heavy bases are 
represented by soda and potash. Texture is equally 
variable, massive agglomerates containing huge volcanic 
bombs marking one extreme, and glassy lavas or dust-like 
tuffs the other. Some slight changes have affected even 
the least altered of the Uriconian rocks. Rhyolites (lava- 
flows) originally in the state of glass, are now devitrified, 
so that under the microscope they present a minutely 
crystalline structure. Fine-grained volcanic muds have 
become extremely hard, and ring under the hammer like 
flint. Both of these forms of alteration may be studied in 
the Uriconian rocks of Little Malvern. Basic lavas and 
ashes have often undergone partial decomposition, the 
hornblende or augite being changed to chlorite, a soft 
green mineral; and if the decomposition has been accom- 
panied by pressure, a chlorite-schist may be produced. 
But in the British area the Uriconian rocks have not 
usually been transformed into crystalline schists, that 
change having much more frequently affected the Mal- 
vernian. | 
And now as to the life of the Uriconian epoch. Un- 
fortunately we know of none. Yet the conditions were 
sometimes favourable to the preservation of organic 
remains. Deposits of volcanic ash laid down under water 
might have entombed the shells of molluscs or the tests 
of crustaceans, and retained them, or impressions of them, 
to all time. The alteration which the Uriconian rocks 
have undergone is often too slight to have obliterated such 
remains. We know that ancient volcanic strata are some- 
times fossiliferous. A well-known example is the Bala 
