184-6.] 



DARWIN ON THE FALKLAND ISLANDS. 



273 



imperfectly crystallized. The quartz-rock must obviously have 

 been in a pasty condition, when it suffered without fracture such 

 abrupt curvatures; and it was impossible to examine these veins 

 without recognising in them the effects of the stretching, and in the 

 fibres or imperfect crystals of quartz, the adhesive nature of the 

 ductile mass *. This hill, as well as the two others in the range, show 

 traces of a qudqudversal or dome-shaped stratification ; and we can 

 thus understand the occurrence of some few veins at right angles 

 to those numerous ones in the line of the principal curvature ; for 

 there must have been some stretching in two directions. I may add 

 that the arched strata in the more regularly dome-shaped hill before 

 described (No. 2), were intersected by a rectangular network of simi- 

 lar veins, almost equally numerous in both directions. All these 

 greatly arched masses of quartz are very brittle. 



Referring once again to the fragment last figured (No. 5), it is seen 

 to be divided by interrupted lines of stratification, concentric with the 

 outer and convex and now accidentally exposed surface, but firmly 

 united together. Captain Sulivan however found in another place 

 innumerable similar fragments, in which the concentric layers were 

 separate, so that the ground was strewed with gigantic semi-cylinders 

 of quartz, like draining or ridge tiles ; he 

 measured one, represented in the diagram 

 annexed (No. 6) and found it twenty feet in 

 length, with a nearly regular diameter of 

 twelve feet. In this instance the edges or 

 rim on both sides are of equal thickness ; 

 but in some other cases, whilst the rim on 

 one side was two feet thick, on the other it 

 thinned off to a knife-edge, evidently in con- 

 sequence of the unequal pressure it had un- 

 dergone. 



Crossing a wide valley of slate and sand- 

 stone we come to the chief mountain-axis 

 of the island, varying from 1500 to 2500 

 feet in height, and running nearly east and 

 west. The strata on its northern flank dip 

 northward ; on the summit, which is from 

 one to two miles broad, they are horizontal ; 

 on its southern side they are almost vertical 



with a southerly dip, and with their summits close to the horizontal 

 beds abruptly arched ; so that in this main range we have the same 

 peculiar form of elevation, so common in all the smaller hills. At 

 the southern base the strata were in some places folded in the 



6. 



* In a paper by M.Elie de Beaumont read before the Soc. Philomatliique, May 

 1839 (L'Institut, 1839, p. 161), it is stated that M. Gaudiu was able to draw out 

 threads of melted quartz : M. Gaudin also found that quartz (differently from alu- 

 mina) retained its viscidity for some time when cooling, — a fact to be borne in 

 mind when we attempt to account for the remarkable flexures which nearly all 

 the quartzose ranges in this island, and likewise in many other i)arts of the world, 

 have undergone. 



1 2 



