VOLCANIC TUFA. 273 



of bitumen which it contains : by any other mode of formation, the 

 bitumen would have been consumed. By some former writers it has 

 been supposed that the tufa is an alluvial bed of sediment and water 

 worn fragments ; but the bituminous nature of this bed excludes the 

 probability of this mode of formation ; and at Monladoux, the up- 

 per part of the tufa may be clearly seen, passing into basalt. In 

 some situations, however, the tufa has been transported from its orig- 

 inal place and intermixed with fragments of more ancient rocks. 



The dome-shaped hills without craters, composed of volcanic por- 

 phyry or trachyte, have given rise to much speculation respecting 

 their origin. Some geologists contend that they are only the remains 

 of one vast bed of trachyte, of which the other parts are washed 

 away. Others maintain that they are merely portions of the granite 

 on which they rest ; and that this granite has been wholly or partial- 

 ly fused, and upheaved, by the expansive force of subterranean fire. 

 This mode of formation is rendered probable, by what may be ob- 

 served at the Puy de Chopine, which is a mountain standing within 

 a crater ; this mountain is composed partly of unaltered granite and 

 sienite, and partly of volcanic trachyte, and appears to have been 

 upheaved, before the fusion of the granite had been effected. 



The Puy de Dome, near the summit, is chiefly composed of whi- 

 tish trachyte intermixed with unaltered granite ; the lower part of the 

 mountain is covered with scoriaceous and compact lava. The dome 

 of this mountain rises 2000 feet above the elevated granitic plain on 

 which it stands, and 4797 feet above the level of the sea : it has no 

 crater or opening on the top ; but Dr. Daubeny says, two streams of 

 lava appear to have pierced the sides of the mountain, and to have 

 descended into the valleys. In this respect the Puy de Dome re- 

 sembles the enormous dome of trachyte on the summit of Chimbo- 

 razo, twenty thousand feet above the level of the sea, which, accord- 

 ing to Humboldt, acts mechanically on the neighbouring country, frac- 

 turing the strata, and changing the surface of the soil ; but it has no 

 permanent opening, either on its summit or sides. In some of 

 these dome-shaped hills, the action of subterranean heat appears to 

 have been so intense, as to reduce the whole into a spongy pulve- 

 rulent mass ; but, what is remarkable, in the middle of this spongy 

 mass, lumps of scoriaceous lava, are sometimes found. It has been 

 objected to the formation of trachyte or volcanic porphyry from gran- 

 ite, that it contains a very small portion of quartz ; but in this res- 

 pect it resembles many granite rocks in Auvergne, in which the 

 quartz is scarcely perceptible. 



In the volcanic districts south of Clermont, the porphyry becomes 

 more compact, and assumes the hardest state of that rock ; the base 

 of the stone is sometimes green, and the crystals of felspar are 

 white : it will receive a fine polish, like the green porphyry of the 

 ancients. 



35 



