42 THE VOYAGE OF H.M.S. CHALLENGER. 



most important mass in the island occurs, Green Mountain, of which we have already- 

 spoken. It includes, besides the peak to which allusion has just been made, several 

 pretty high summits. Weather Post Hill (1965 feet) is situated towards the east, and 

 a little farther south there is a large depression in the form of an elongated ellipse which 

 bears the name of Cricket Valley. Booby Hill 1 (1790 feet), to the south of the valley 

 which borders the central heights of Green Mountain, is also associated with that mass. 

 In the same central region, but more to the west, is Riding School Crater, and still 

 farther west Red Hill. Cross Hill is situated near the village of Georgetown. We 

 have now enumerated and stated the position of the principal hills which will be referred 

 to in this Report. 



Augitic Trachytes. 



We have said that trachytic rock forms the fundamental mass of the island, and we 

 shall commence the description of the rocks by that of the trachytic type, giving first, 

 according to Darwin, 2 the macroscopic characters. They occupy the highest and most 

 central part of the island, and also occur in the south-east region. This trachyte is 

 usually of a pale brown colour, speckled with black spots ; it contains folded and broken 

 crystals of glassy felspar, grains of hematite, and black microscopic particles which 

 Darwin referred, doubtfully, to hornblende. The greater number of the eminences are 

 formed of a white friable rock. 3 Obsidian, hornstone, and several other zonary felspathic 

 rocks are associated with the trachyte. The last-named is never stratified, nor are 

 crater-formed orifices ever found on the eminences. The trachytic region must have 

 been violently dislocated ; the fissures are still open, or partially filled with loose 

 fragments. The space occupied by these trachyte masses is bounded by a line which 

 surrounds Green Mountain and joins the hills of " Weather Post Signal " and " Crater 

 of an old volcano." Trachyte predominates in the region thus circumscribed ; it is 

 traversed by some veins of basalt, and near the summit of Green Mountain there is a 

 stratum of vesicular basalt enclosing crystals of glassy felspar with rounded outlines. 



The soft white rock mentioned above bears a close resemblance to a sedimentary tufa 

 when seen in the mass. Darwin hesitated for some time, as many other geologists have 

 done in analogous cases, before he rejected this theory of its origin. He observed, on 

 two separate occasions, that the white earthy rock formed isolated hills ; in another 



1 Dr. Maclean points out in a manuscript correction of Bedford's chart, of -which we avail ourselves, that the name 

 "Booby Hill" should be substituted for "Red Hill." The latter name should be given, as I say in the text, to the 

 hill west of Riding School Crater. The rocks described in this Report as coming from Red Hill were collected by 

 Dr. Maclean, and were obtained from the hill situated in the position which he has marked on the map. 



2 Darwin, Geological Observations, pp. 42-44. In summarising passages we have preserved as much as possible 

 the mineralogical and petrographical nomenclature, and the interpretation of facts given by the author. One might, 

 in some cases, be able to modify them, but this would involve the risk of making more or less arbitrary changes, since 

 we have not the specimens Darwin employed to refer to. 



3 It may be that in certain cases the rock spoken of by Darwin as whitish trachytic tufa is siliceous earth, as in 

 the case of the whitish deposits of Riding School Crater. 



