Fenton — Phjsiographj and Glacial Geology of B. Patagonia. 193 



apparently conformably on typical tertiary rock. On examining this rock, 

 however, it is found not to consist of pure shingle, as one would imagine from 

 having seen the similar formation in other places, but to have a composite 

 structure. The shingle is found above as elsewhere, but below this there is 

 another layer of equal thickness of marine formation holding an abundance 

 of fossilized sea-shells. This marine layer has been desciibed by Mr. J. B. 

 Hatcher, and named by him the Cape Fairweather Bed. The unconformity 

 between the Cape Fairvveatlier Bed and the tertiary rock is slight but 

 distinct, whereas there seems to be in places none between the Cape Fair- 

 weather Bed and the shingle, the two layers passing imperceptibly into one 

 another. In no instance in the district in question have I yet found any 

 formation overlying this shingle except lava ; the latter exists, however, in 

 extensive sheets in many localities, and will be dealt with in a later section. 



We have already seen that the apparent unconformity between the 

 shingle and the tertiary rock is on the highest level always insignificant ; it 

 will consequently be evident that, even if all the shingle were removed, the 

 pampa would still possess that extraordinary flatness which I have mentioned 

 as existing everywhere, and it will be further evident, moreover, that this 

 flatness must have been a primeval condition, and must have existed before 

 the shingle appeared. It would seem also that Lhe shingle crept down gently 

 and gradually over this smoothed and level pampa until it completely 

 covered it, and that the transition from the condition of the climate which 

 brought al)out the planing off of the pampa to those which covered the latter 

 with sand and shingle was completely devoid of violence. The tertiary rock 

 underlying the Cape Fairweather Bed has been generally looked upon as 

 belonging to the Miocene system ; this would place the Cape Fairweather Bed 

 either in the late Miocene, the Pliocene, or early Pleistocene. The uncon- 

 formity between the Cape I'airweather Bed and the tertiary rock is, as we 

 have seen, slight ; but it does not follow necessarily from this that there was 

 a short hiatus in time between the periods when they were respectively 

 deposited. It may only mean that the great planing ofl' process extended 

 much further back into tertiary times than we might have thought, and 

 was operating before the Cape Fairweather Bed was formed. We have 

 evidence from other sources that the tertiary rock suffered considerable 

 erosion before the shingle came down, and that it once extended to a much 

 greater altitude ; it would consequently seem that the elevation and sub- 

 sidence which occurred during tertiary times took place very gradually. 

 This is particularly clear when we remember the complete absence of faulting 

 and folding found in these rocks, and the perfect horizontality of the strata 

 composing them. All these facts would point to a condition of affairs in pre- 



