406 TERAI. Chap. XVII. 



table mould, nowhere so thick and rich as in the more humid 

 regions of 7000 feet elevation. The rocks appeared in the 

 following succession in descending. Along the top are 

 found great blocks of very compact gneiss buried in clay. 

 Half a mile lower the same rock appears, dipping north- 

 north-east 50°. Below this, beds of saccharine quartz, with 

 seams of mica, dip north-north-west 20°. Some of these 

 quartz beds are folded on themselves, and look like flattened 

 trunks of trees, being composed^of concentric layers, each 

 from two to four inches thick : we exposed twenty-seven feet 

 of one fold running along the side of the road, which was cut 

 parallel to the strike. Each layer of quartz was separated 

 from its fellows, by one of mica scales, and was broken up 

 into cubical fragments, whose surfaces are no doubt cleavage 

 and jointing planes. I had previously seen, but not under- 

 stood, such flexures produced by metamorphic action on 

 masses of quartz when in a pasty state, in the Falkland 

 Islands, where they have been perfectly well described by 

 Mr. Darwin ; * in whose views of the formation of these 

 rocks I entirely concur. 



The flexures of the gneiss are incomparably more irregular 

 and confused than those of the quartz, and often contain 

 flattened spheres of highly crystalline felspar, that cleave 

 perpendicularly to the shorter axis. These spheres are dis- 

 posed in layers parallel to the foliation of the gneiss : and are 

 the result of a metamorphic action of great intensity, effect- 

 ing a complete rearrangement and crystallization of the 

 quartz and mica in parallel planes, whilst the felspar is aggre- 

 gated in spheres ; just as in the rearrangement of the mineral 

 constituents of mica-schists, the alumina is crystallized in 

 the garnets, and in the clay-slates the iron into pyrites. 



* Journal of Geological Society for 1846, p. 267, and " Voyage of the Beagle." 



