440 Falkland Islands. part n. 



that it is of mechanical and sedimentary origin ; yet 

 it thinned out upwards, and did not cut through the 

 uppermost strata in the cliffs. This fact at first appears 

 to indicate that the matter could not have been washed 

 in from above: l but if we reflect on the suction which 

 would result from a deep-seated fissure being formed, 

 we may admit that if the fissure were in any part open 

 to the surface, mud and water might well be drawn 

 into it along its whole course. The third dike consisted 

 of a hard, rough, white rock, almost composed of broken 

 crystals of glassy feldspar, with numerous scales of black 

 mica, cemented in a scanty base ; there was little in 

 the appearance of this rock, to preclude the idea of its 

 having been a true injected feldspathic dike. The 

 matter composing these three pseudo-dikes, especially 

 the second one. appears to have suffered, like the sur- 

 rounding strata, a certain degree of met amorphic 

 action ; and this has much aided the deceptive appear- 

 ance. At Bahia, in Brazil, we have seen that a true 

 injected hornblendic dike, not only has suffered meta- 

 morphosis, but has been dislocated and even diffused 

 in the surrounding gneiss, under the form of separate 

 crystals and of fragments. 



Falkland Islands. — I have described these islands 

 in a paper published in the third volume of the ' Geo- 

 logical Journal.' The mountain-ridges consist of quartz, 

 and the lower country of clay-slate and sandstone, the 

 latter containing palaeozoic fossils. These fossils have 

 been separately described by Messrs. Morris and Sharpe : 

 some of them resemble Silurian, and others Devonian 

 forms. In the eastern part of the group the several 

 parallel ridges of quartz extend in a west and east line; 



1 Upfilled fissures are known to occur both in volcanic and in 

 crdinaiy sedimentary formations. At the Galapagos Archipelago 

 (see Chapter Y. of this work), there are some striking examples of 

 pseudo-dikes composed of hard tuff 



