470 



ME. 0. A. DEKBY ON 



metres above the general level. Similar but shorter detached ridges 

 (the Serra do Caracol and Serra de Caldas) to the southward and 

 eastward appear to complete an approximately circular or elliptical 

 enclosure (fig. 7). This circular arrangement of the higher ridges 

 is peculiar, and, taken in connexion with the character of the pre- 

 dominant rocks of the plateau and of the bounding ridges, as far as 

 examined, is probably not without significance. 



Pig. 7. — Sketch Map of the Caldas region. 



In the 18 kilometres of railway from Cascata to the village of 

 Pogos the cuttings are mainly in decomposed rock, which is, however, 

 sufficiently preserved in patches to show its original character, either 

 in loose masses or in the traces of structure still visible in the clay 

 resulting from decomposition. The predominant rock is phonolite. 

 Foyaite appears rarely in a few cuttings, always totally decomposed. 

 No evidence of the existence of other rocks was met with, and the 

 absence of dykes is noticeable. About 4 kilometres from Pogos two 

 cuttings in decomposed phonolite show an abundance of decomposed 

 analcime, some of the crystals measuring 3 inches in diameter. 

 Near Cascata two quarries, already referred to, have been opened 

 to the right of the road in hills of foyaite with abundant inclu- 

 sions. Close to one of these there is a hill of coarsely porphyritic 

 phonolite unlike any variety seen on the railway. The crystalline 

 inclusions are of all sizes up to an inch, in diameter, with ill-defined 

 limits and generally stained red, the red colour extending at times 

 to the ground-mass, which is normally bluish, the inclusions being 

 white. This reddening of the rock without visible signs of decay 

 has extended to nearly the whole quarry, so that masses free from 



