AGE OF VOLCANIC ROCfcS. 



281 



ftdmitted, and it is desirable to separate theoretical views from a de* 

 scription of facts. This, however, cannot always be done : circum- 

 stances which indicate the mode of rock formations, will deservedly 

 force themselves on our attention *, and in stating them fairly, and 

 the inferences which may be drawn from them, we relieve geology 

 from much of its dryness, and stimulate succeeding observers to a 

 strict investigation of nature. 



Dark-coloured recent lava does not differ essentially from basalt ; 

 it is generally more porous. Probably the compact state of basalt 

 was the result of refrigeration under pressure ; it may, however, be 

 frequently observed in Auvergne, passing into the state of scoriace- 

 ous lava. Some of the recent lavas from Vesuvius are compact, 

 and have a glistening lustre, but they are more commonly porous. 

 In some volcanic eruptions, lava appears to have acquired the most 

 perfect fluidity. According to Professor Boltis, who was an eye- 

 witness of the eruption of Vesuvius in 1776, the lava spouted from 

 three small apertures, precisely like water, forming beautiful foun- 

 tains of fire, which described curves of different dimensions as they 

 fell. In the same year, a current of lava from the summit of Vesu- 

 vius flowed with the velocity of a mile and a half in fourteen min- 

 utes; it struck upon the lava of 1771, and rebounded into the air, 

 congealing in figures of various shapes. The length of time which 

 currents of lava retain their heat is truly remarkable : the current 

 which flowed from jEtna in 1669 is two miles in breadth, fifteen 

 miles in length, and two hundred feet in depth ; it retains a por- 

 tion of its heat to the present day. Ferrara says, when this lava 

 was perforated at Catania in 1809, flames broke out 5 and it contin- 

 ued to smoke at the surface after rain, at the beginning of the pres- 

 ent century, or 130 years after its eruption. 



Stones of enormous size are frequently projected from the craters 

 of volcanoes ; but the quantity of matter which they throw out in the 

 state of scoriae, sand, and powder, often exceeds that erupted in the 

 state of lava, and is spread over distant countries. By the percola- 

 tion of water it becomes agglutinated, and forms beds of volcanic 

 breccia, and tufa. Sometimes the tufa is sufficiently solid to be used 

 for building-stone ; the Roman pepperino is a volcanic tufa. Poz- 

 zolana consists of minute particles of scoriae, which have been par- 

 tially decomposed : when mixed with lime, it makes a water-setting 

 cement. 



Some volcanic rocks decompose rapidly, and form productive 

 soils ; others resist the process of decomposition so effectually, that, 

 after the lapse of some thousand years, they present all the freshness 

 of the most recent lavas. 



»^ge of Volcanic Rocks. — Nothing precise can be determined with 

 respect to the relative age of volcanic rocks, except in those districts 

 where they occur together, one covering the other. Humboldt, who 

 has attempted to trace the different ages of volcanic formations, ob- 



36 



