Volcanic Islands visited during the Voyage of the Beagle. 355 



and if the rate of cooling and nature of the mass had been favourable to 

 its crystallization or to concretionary action, we should here have had the 

 finest parallel zones of different composition and texture. In glaciers, the 

 lines of porous ice and of minute crevices seem to be due to an incipient 

 stretching, caused by the central parts of the frozen stream moving raster 

 than the sides and bottom, which are retarded by friction ; hence, in gla- 

 ciers of certain forms and towards the lower end of most glaciers, the zones 

 become horizontal. May we venture to suppose that in the felspathic 

 lavas with horizontal lamina? we see an analogous case? All geologists 

 who have examined trachytic regions have come to the conclusion, that 

 the lavas of this series have possessed an exceedingly imperfect fluidity; 

 and as it is evident that only matter thus characterized would be subject 

 to become fissured and to be formed into zones of different tensions, in the 

 manner here supposed, we probably see the reason why augitic lavas, which 

 appear generally to have possessed a high degree of fluidity, are not*, like 

 the felspathic lavas, divided into laminae of different composition and tex- 

 ture. Moreover, in the augitic series, there never appears to be any ten- 

 dency to concretionary action, which we have seen plays an important part 

 in the lamination of rocks of the trachytic series, or at least in rendering 

 that structure apparent." 



In the fourth chapter, relating to St. Helena, we have by far the 

 most definite and explicit account of the structure of that island 

 which has yet been made public, and from which, if studied in 

 conjunction with the views and local descriptions given in Scale's 

 " Geognosy " of the island, the geologist may acquire positive and 

 satisfactory notions of the phaenomena it presents, such as will be 

 comparable with those which he possesses of the volcanic regions of 

 Naples or Central France. We would gladly follow our author in 

 this place, but our limits warn us to conclude ; and simply referring 

 to the account, p. 87, of certain beds of soft calcareous sandstone in 

 St. Helena, the particles composing which have been drifted into their 

 present position by the wind, and subsequently consolidated by the 

 percolation of rain-water, we pass to the end of the chapter, from 

 which we extract the following instructive considerations on a difH- 

 cult and much-debated subject. 



" Craters of Elevation. — There is much resemblance in structure and in 

 geological history between St. Helena, St. Jago, and Mauritius. All three 

 islands are bounded (at least in the parts which I was able to examine) by ' 

 a ring of basaltic mountains, now much broken but evidently once con- 

 tinuous. These mountains have, or apparently once had, their escarp- 

 ments steep towards the interior of the island, and their strata dip out-- 

 wards. I was able to ascertain, only in a few cases, the inclination of the 

 beds ; nor was this easy, for the stratification was generally obscure, ex- 

 cept when viewed from a distance. I feel, however, little doubt that, ac- 

 cording to the researches of M. Elie de Beaumont, their average inclination 

 is greater than that which they could have acquired, considering their thick- 

 ness and compactness, by flowing down a sloping surface. At St. Helena 



* " Basaltic lavas, and many other rocks, are not unfrequently divided into 

 thick laminae or plates of the same composition, which are either straight 

 or curved; these being crossed by vertical lines of fissure, sometimes be- 

 come united into columns. This structure seems related in its origin to 

 that by which many rocks, both igneous and sedimentary, become traversed 

 by parallel systems of fissures." 



2 B2 



