612 Mr. J. Y. Buchanan on Geological Work 



The effect of heat in rendering many minerals which occur in igneous 

 rocks decomposable by acids is well known, and may furnish an ex- 

 planation of the above-named phenomena. As the dykes extend to the 

 very tops of the hills, and the adjacent rock is weathered away leaving 

 them projecting, the outline of the hills gets a peculiar serrated appear- 

 ance, which is very characteristic of the island. The spurs which project 

 from the main ranges into the plains are always found to depend for their 

 height and direction on one of these basaltic dykes, which can be followed 

 all the way along their crest. 



In many of the rocks, where their character is cellular, their cells are 

 filled with geodes, in some cases of carbonate of lime, and in others of 

 zeolite. The rocks are nearly all of a porphyritic character from crystals 

 of augite separated out. The most common rock contains large augites 

 in a light grey ground-mass. The crystals are frequently arranged in 

 bands with their longer axes parallel to each other and in the direction 

 of the bands. Nothing approaching to either obsidian or pumice was 

 observed in any part of the island. Besides the igneous rocks, there is 

 in the plain on which the town stands and to the south of it a calcareous 

 formation consisting of shells, corals, and rounded pebbles of the neigh- 

 bouring rocks, united by a firm paste of a white and sometimes of a 

 reddish colour ; also, from the constant prevalence of winds from one 

 quarter, the sand from the beach has been distributed over the plains 

 and in the valley. At the south side of the harbour there are dunes of 

 blown sand, which in some places is consolidated into rocks similar to 

 those of Bermuda. Here, however, the sand does not consist purely of 

 carbonate of lime, but of the debris of shells and volcanic material mixed. 

 Hence the consolidation of it into rock is not so easy, even if there was 

 the quantity of rain to do it. On the sand hills the wind produces the 

 usual beautiful ripple-mark, and the mixed character of the sand makes 

 the levigating action of the wind very apparent, the light calcareous 

 particles being swept into the eddies under the crest of the ripple, the 

 heavy volcanic debris remaining on the surface. As the colour of the 

 latter is generally black and that of the former white, a sand hill looks 

 lighter or darker according as it is viewed from the weather or from the 

 lee side. 



"We were told that on the other side of the island there were tolerably 

 perfect craters ; we were, however, unable to visit them. In the neigh- 

 bourhood of the harbour there was no vestige of recent volcanic action. 

 At St. Paul's rocks the ship was made fast to the rocks, and as we re- 

 mained there over thirty-six hours we had ample time to thoroughly 

 explore them. They were visited by Darwin in the ' Beagle,' and several 

 times later by other ships. As the nature of the rocks appears to pre- 

 sent some difficulty to geologists, I made as complete a collection of 

 them as I could, and they will be sent home with the other collections 

 from this place. The white enamel-like incrustation mentioned by 



