Volcanos. 131 



" The observed instances of eruptions from the sea are indeed 

 few in number. Our author mentions those off St. Michael, one 

 of the Azores in 1638, 1720, and J811 ; ofSantorini, and the Isola 

 Nuova, in the Archipelago; another off the coast of Iceland in 

 1782; and one amongst the Aleutian islands in 1814. But, on 

 reflection, we must conclude, that the weight of the water above 

 the vent, and the refrigerating effect of its contact, must, in all 

 cases, condense the escaping volumes of steam, and prevent their 

 rising to the surface, and rendering the eruption visible there, 

 except when the orifice of the volcano has been raised by the 

 accumulated products of repeated eruptions, to within a short 

 distance of that level ; so that numerous eruptions may be con- 

 tinually taking place within the depths of the ocean, without our 

 being aware of their occurrence in any way. There is no rea- 

 son for concluding such eruptions to proceed in any very different 

 manner from those which are subaeriaL The expansive force 

 and temperature of the lava must be extreme, and proportioned 

 to the great excess of the repressive force occasioned by the 

 pressure of the supported column of water. The lavas, when 

 emitted, will therefore, from the intensity of their temperature, 

 and the resistance opposed by this dense medium, to the exu- 

 dation of the confined vapor, retain their fluidity much longer 

 (than ?) in the open air, and consequently, spread laterally to a far 

 greater distance from the vent, with a similar inclination of sur- 

 face. According to this, lava beds, produced at the bottom of 

 the sea, ought to exhibit a greater lateral extension, compared 

 with their bulk, than those which have flowed from subaerial 

 volcanoes ; and, in fact, the great horizontal dimensions of the 

 floetz-trap formations of Ireland, Germany, Iceland, Faroe, the 

 Hebrides, &c. have long been a subject of remark. Again, 

 since little or no vapor can escape from the suface of the lava, 

 such beds should show very few scoriae or scoriform parts on 

 their upper surface ; and, on the contrary, vesicles, or air cells, 

 may be expected often to abound through the interior of the 

 rock, the extreme tension of the steam causing its parcels to 

 expand as the lava flows on, while the rapid consolidation of 

 the surface, and the weight of the sea above, must prevent 

 their rising upwards. 



These characters also accord with the appearances of many 

 of the flcetz-trap rocks, amygdaloids, &c. which seem clearly 

 to be the products of submarine vents. Of the fragments thrown 

 up by the explosions of submarine eruptions, some will accumu- 

 late round the orifice in rude beds, others be dispersed by cur- 

 rents, and mixed or interstratified with other marine deposits. 

 In the north of Italy and Sicily, are frequent examples of cal- 

 careo-basaltic conglomerates, (peperino.) as well as of beds of 



