320 Notes, chiefly Geological, [No 172. 



chain : a few angular blocks of a large grained, and a syenite, granite 

 were also seen. 



The descent to the foot of the pass is about six miles, and extremely 

 steep. At its upper extremity I observed in a road section, first a thin 

 layer of dark vegetable mould, then a thick bed of red clay, under 

 which lay a stratum of laterite. Farther down in the pass, gneiss out- 

 cropped. Some of the cavities, in the laterite, contained a black bole. 



Fragments of white quartz, imbedding large crystals of felspar, often 

 pinkish, were picked up imbedding a silvery-white mica, in large plates. 



Farther down the pass I did not see the laterite. Hornblende schist 

 with garnets of a massive thick-bedded structure was the prevalent 

 rock. This had often been blasted to improve the road ; and the beds 

 of clay, which covered it, had been removed, exhibiting the different 

 stages of weathering which this rock undergoes. 



Blocks of this kind not only often exhibit a concentric structure 

 like that of granite, but still oftener a pseudo internal structure, from 

 weathering internally in layers conformable to their exterior surfaces. 



Fragments, several feet in diameter, are seen thus weathered ; with 

 nothing but a dark crystalline nucleus of the rock in its original state 

 in the centre, to tell us what the variegated soft mass before us once 

 was. Even the nucleus disappears before the ravages of this maladie 

 du granit in due time. 



This decay does not commence from the core, but from the exterior 

 of the block, whence it sinks by successive phases from the circum- 

 ference towards the centre. 



The effect of these different stages of decay is to produce, in the 

 substance of the block, differently coloured bands, one within another, 

 (like the lines of agate) often arranged around a nucleus of sound 

 dark crystalline hornblende rock in the centre. The first band around 

 this nucleus is of a grey colour, from the felspar whitening, and the 

 segregation, &c. of the iron, which coloured it. The hornblende 

 crystals are little affected, and the felspar is often seen running among 

 them in whitish reticulation. 



The next band exhibits the rock in a state of greater decay. In 

 this the hornblende crystals have commenced to oxidize; and, without 

 mingling with the felspar, assume an orange-brown hue, still mottled 

 slightly with dark specks This band has a mottled appearance, and 

 resembles a weathered granite. 



