4-1 8 Formation during Subsidence paet n. 



list of these fossils to Professor E. Forbes, he -thinks 

 they must have lived in water between one and ten 

 fathoms in depth : hence the bottom of the sea on 

 which these shells once lived must have subsided at 

 least 700 feet to allow of the superincumbent matter 

 being deposited. I must here remark, that, as all these 

 and the following fossil shells are extinct species, Pro- 

 fessor Forbes necessarily judges of the depths at which 

 they lived only from their generic character, and from 

 the analogical distribution of shells in the Northern 

 Hemisphere; but there is no just cause from this to 

 doubt the general results. At Huafo the strata are 

 about the same thickness, namely, 800 feet, and Pro- 

 fessor Forbes thinks the fossils found there cannot have 

 lived at a greater depth than fifty fathoms, or 300 feet. 

 These two points, namely, Navidad and Huafo, are 570 

 miles apart, but nearly half-way between them lies 

 Mocha, an island 1,200 feet in height, apparently formed 

 of tertiary strata up to its level summit, and with many 

 shells, including the same Turritella with that found 

 at Huafo, embedded close to the level of the sea. In 

 Patagonia, shells are numerous at St. Cruz, at the foot 

 of the 850 feet plain, which has certainly been formed 

 by the denudation of the 840 feet plain, and therefore 

 was originally covered by strata that number of feet in 

 thickness, and these shells, according to Professor 

 Forbes, probably lived at a depth of between seven and 

 fifteen fathoms : at Port St. Julian, sixty miles to the 

 north, shells are numerous at the foot of the ninety feet 

 plain (formed by the denudation of the 950 feet plain), 

 and likewise occasionally at the height of several hundred 

 feet in the upper strata ; these shells must have lived in 

 water somewhere between five and fifty fathoms in depth. 

 Although in other parts of Patagonia I have no direct 

 evidence of shoal-water shells having been buried under 



