THE DESERT SANDSTONE. 323 



will suggest comparisons with those of America. When 

 consolidated and affected by oxidation the sands become brown 

 and red with bands of limonite. 



It is nothing new in geology to identify an extensive rock 

 formation with ash deposits. The ancient city of Rome itself 

 affords an apt illustration of this, which bears so high an interest 

 from its scientific and its antiquarian character that it is well worth 

 the space it will occupy by a reference to it. Everybody has 

 heard of the Seven Hills of Rome, on which the city is seated at 

 the eastern side of the valley of the Tiber. Four of the hills 

 namely the Quirinal, Viminal, Ccelian, and Aventine belong to the 

 rising ground or plateau forming the valley. Two of them, the 

 Palatine and Capitoline are detached. The Palatine, 170 feet 

 high, appears to have had precipitous edges. The Capitoline, 

 though only 150 to 160 feet in height, is from its abrupt face and 

 well-marked outline, a conspicuous instance of a relic of ash-beds 

 spared from the weathering of rain and rivers. For the rising 

 ground on the eastern side, an elevated flat which reaches its 

 culminating point in the Esquiline, 218 feet p^bove the river, is 

 composed of volcanic ashes once spread in a continuous sheet all 

 along the valley. The ashes are now consolidated into a volcanic 

 sandstone or tufa, hard enough to be used as building stone, but 

 also easily excavated into the extensive subterranean cemeteries 

 of the Catacombs. This great deposit of ashes came from some 

 recent craters at no great distance from the city. Seventeen 

 miles or so on the north-west are the Ciminian Hills, with Lake 

 Bracciano, an enormous crater, now filled with water instead of 

 fire. Thirteen miles to the south-east are the Alban Hills, with 

 the relics of another extinct crater at Lake Albano. The 

 Janiculum Hill which is on the west side of the Tiber has but 

 little ash on its north side, but is composed of marine beds with 

 shells such as now exist in the Mediterranean, though many are 

 extinct. On the flanks of the Aventine Hill there is a still later 

 deposit of freshwater travertin or recent limestone, showing that 

 the river reached 140 feet higher than it does at present, before 

 the ash -beds were cut down, and the Mistress of the World had 

 spread herself out on the sides of the valley. She played her part 

 and innumerable ruins tell of her former splendours. But the 

 ash-beds lie beneath all, telling of a phase in her history such as 

 once was shared by Australia. 



It may be mentioned also as an illustration of these ash 

 accumulations, that I saw the result of a very recent eruption at 

 the volcano of Taal, near Manila in the Philippines. I descended 

 into the crater in March, 1885, when all was quiet ; but returning 

 in 1886 I found a singular scene of devastation. An eruption 

 had commenced in the preceding September, and the fall of ashes 



