U- S. Board on 

 Geographical Names 



chap. i. Ancient Volcanic Hills.' f 23 



the stages of decomposition of this latter mineral, and 

 I found, to my surprise, that I could trace a nearly 

 perfect gradation from unaltered olivine to the green 

 wacke. Part of the same grain under the blowpipe 

 would in some instances behave like olivine, its colour 

 being only slightly changed, and part would give a 

 black magnetic bead. Hence I can have no doubt that 

 the greenish wacke originally existed as olivine ; but 

 great chemical changes must have been effected during 

 the act of decomposition thus to have altered a very 

 hard, transparent, infusible mineral, into a soft, unctu- 

 ous, easily melted, argillaceous substance. 1 



The basal strata of these hills, as well as some 

 neighbouring, separate, bare, rounded hillocks, consist 

 of compact, fine-grained, non-crystalline (or so slightly 

 as scarcely to be perceptible,) ferruginous feldspathic 

 rocks, and generally in a state of semi-decomposition. 

 Their fracture is exceedingly irregular, and splintery; 

 yet small fragments are often very tough. They 

 contain much ferruginous matter, either in the form of 

 minute grains with a metallic lustre, or of brown hair- 

 like threads : the rock in this latter case assuming a 

 pseudo-brecciated structure. These rocks sometimes 

 contain mica and veins of agate. Their rusty brown or 

 yellowish colour is partly due to the oxides of iron, but 



1 D'Aubuisson, 'Traits de Geognosie ' (torn. ii. p. 569), mentions, 

 on the authority of M. Marcel de Serres, masses of green earth near 

 Montpellier, which are supposed to be due to the decomposition of 

 olivine. I do not, however, find, that the action of this mineral 

 under the blowpipe being entirely altered, as it becomes decom- 

 posed, has been noticed ; and the knowledge of this fact is im- 

 portant, as at first it appears highly improbable that a hard, 

 transparent, refractory mineral should be changed into a soft, 

 easily-fused, clay, like this of St. Jago. I shall hereafter describe 

 a green substance, forming threads within the cells of some vesicular 

 basaltic rocks in Van Diemen's Land, which behave under the blow- 

 pipe like the green wacke of St. Jago ; but its occurrence in 

 cylindrical threads, shows it cannot have resulted from the decom- 

 position of olivine, a mineral always existing in the form of grains 

 or crystals. 



