1^3 Volcanos. 



their parallel plane surfaces and null in the transverse direction. 

 Hence, retreat fissures will be produced in abundance, parallel 

 to the largest plane surfaces of the crystals, and few or none will 

 be formed transverse to these. By the frequency of such trans- 

 verse fissures, the cubical or rhomboidal structure is produced. 



"The author next touches on the question, as to the cause of 

 the difference of mineral composition in lavas. He inclines to 

 attribute this variety to certain alterations undergone by the rock, 

 originally of an uniform (perhaps granitic) composition, during 

 its rise to the surface of the globe ; which was probably attend- 

 ed by repeated alternations of intumescence and reconsolidation, 

 from changes in the relative proportions of the intense heat and 

 pressure to which it was subjected. The principal varieties of 

 lava are found in nature to have been usually produced success- 

 ively often alternately, from the same or proximate vents. The 

 opinion of the antagonism of trachyte and basalt, put forth by 

 Humboldt and Beudant, is combated by our author, and numer- 

 ous examples adduced of their successive emission from the same 

 volcano. He then notices on the error of limiting the produc- 

 tion of trachyte or basalt to particular ages of the globe, or 

 making "formations" of them — both are produced before our 

 eyes by recent and still active volcanos ; the error arises from 

 the terms trachyte and basalt, not having been confined, as of 

 right to a mineralogical meaning. 



" Having thus far traced the laws which determine the dispo- 

 sition of the substances produced by a single volcanic eruption, 

 whether in a fragmentary form, or as more or less liquid lava, 

 the author proceeds to examine the circumstances that result 

 from the accumulation of such products, by repeated eruptions 

 from the same vent. The simple cone by this process, becomes 

 enlarged into a volcanic mountain composed of hardened lava- 

 streams, (each of which acts like a solid rib or buttress to the 

 hill,) and intervening beds of conglomerate. The sides of this 

 hill, are frequently, during eruptions, split by the pressure of 

 the column of lava, within the central aperture or chimney of 

 the volcano. The lava then flows out through orifices, formed 

 successively at different levels, one below the other ; examples 

 of such occurrences are given from the phenomena of iEtna, 

 Vesuvius, Iceland, &c. It is a general fact, that, in the erup- 

 tions of volcanic mountains, or habitual volcanos, the elastic fluids 

 are chiefly discharged from the central crater, but the Java is emit- 

 ted from apertures in the side, or at the base of the mountain. 

 Minor and local earthquakes are occasioned by these rendings of 

 the frame-work of the mountain, which is even sometimes split 

 in two. The consolidation of the lava that occupies these fis- 

 sures, produces numerous vertical dikes, which cutting across its 



