OBSTRUCTION OF DRAINAGE 339 



poured down the slopes, as may be well seen on the 

 southern flanks of the Bracciano volcano. 



In most instances the molten rock stopped short 

 on the flanks of its parent mountain, but it occasionally 

 descended into the plain, as in the familiar example 

 on the Via Appia, where the stream flowed from the 

 side of the Alban volcano for some six miles to 

 within a short distance of the site of the future capital 

 of the world. The lava is here a firm, compact, 

 durable stone admirably adapted for pavements, a use 

 to which it has been extensively put for more than 

 two thousand years, both within the walls and on 

 the great high roads that radiate from them. Here, 

 again, we see how bountifully Rome was favoured in 

 regard to the materials needed for the construction 

 of a great city. 



The heaping of so much volcanic rock over the 

 surface of the country must have greatly modified 

 its topographical features. The drainage would espe- 

 cially be affected. Streams descending from the 

 Apennines would find their direct passage to the sea 

 blocked by the newly formed ridges, hills and moun- 

 tains, and they might have to make long circuits 

 before finding an exit. Lakes would gather in the 

 hollows of the irregularly deposited tuff and others 

 would fill up the cavities blown out by explosions, so 

 as to become crater-lakes. The case of the Tiber may 

 be cited in illustration of the deflection of drainage. 

 In earlier times, as I have already remarked, this 

 river probably flowed southwestwards across the site 

 of the volcanic district of Bolsena and Viterbo ; but 

 in consequence of the subsequent eruptions, the lower 



