22 



rock, which is nearly buried in the soil, were exposed to meteoric action, a deep groove 

 woold soon be formed in the line were the laminae part and bend inwards. 



The metallic vein formerly noticed runs NE. by E. The whole rock is traversed by other 

 planes in the same direction, as appears from slight scorings on the surface. Another princi- 

 pal divisional plane is SE. by S. as is well seen by the direction of the edges of some large 

 rocks adjoining. 



To the SW. of the grooved rock first described, the rocks on the beach are either extended 

 and flat, and a few feet above the level of the beach,— or, were worn down nearly to the 

 level of the beach, they are broken into small cuboidal and spherical fragrnents disposed, 

 where this has been recently done, in regular lines. It is obvious how readily rocks with 

 such a structure may be worn into cavities and channels. 



Having now sufficiently gone into details for the scope of this paper, it remains to explain 



the conclusions to which I have been led by my observations. In truth, however, I have 



not much to say on this subject (save what is of a general speculative nature) that has 



not been anticipated in the second portion of the preceeding remarks. The first circum- 



stance worthy of note is, that the observer after he has partially explored the Island is 



wholly unable to conjecture , at many of the Points , whether the next few paces along 



the shore will bring him to a granific, a syenitic, a dioritic , or a basaltic rock , or even 



whether the mass before him , although at the place where he has broken off a fragment 



decidedly a granite , may not in other places be found to consist of any or all of these 



other minerals. The Island , in its general mineralogy , seems to break through all arbi- 



trary distinctions of plutonic and volcanic , and to confirm , in a very striking raanner , 



the conclusion , to which most geologists have arrived , that these great classes of rocks are 



essentially similar in origin. The difference in structure, as between a compact basalt and 



a crystalline granite , is referred to the difference in pressure to which the basalt , cooling 



near the surface, and the granite, cooling at great depths , were subject at the time of 



.... 

 their solidification. That this may , or rather must have a great influence is certain ; but 



the occurrence of such rocks as these of Pulo Ubi?i cannot be explained by difference of 

 pressure , and , indeed , proves that there are in nature causes independent of variations in 

 pressure , adequate to the contemporaneous production of rocks belonging both to the volca- 

 nic and the plutonic series (1). Under whatever circumstanccs the granite of the Island 

 was produced, under the same circumstances were the syenites and greenstones also produced; 

 and some other cause than great difference of depth and consequent pressure must have 

 determined the mutations in the mineral character of the mass. But if it be certain that 

 such cause existed , would not that cause of itself be adequate as a general origin of the 

 differences in igneous rocks attributed to inequalities of pressure ? Mr. Lyell (perhaps the most 

 distinguished of our English writers on geology , whether we consider the originality of his 

 views , the philosophical spirit in which they are generally conceived , or the graceful sim- 



(1) Mr. LïEIl confines the term plutonic to granites (Elements vol. I. p. 15 and vol. II. Chap. 32.) and I here 

 use it in the same sense. Olher writers, and perhaps Mr. Lteii. himself occasionally , give it a wider meaoing. 



