Researches on the Mineralogy of South America. 135 



have at once a surface of equilibrium ; and the laws of hydro- 

 statics show that it must be a spheroid. But we have no means of 

 actually measuring the form of the ocean. Were it studded with 

 islands so that a large part could be covered with survey-triangles, 

 this might be remedied. In the absence of this we are obliged to 

 confine ourselves to the continents ; and by supposing them per- 

 meated by canals letting in the sea-water to any extent, calcula- 

 ting by means of the spirit-level how it would lie, we ascertain 

 by laborious surveys that the canals lie very nearly on a sphe- 

 roidal surface. This surface, where it comes in contact with the 

 sea at sea-coast stations, of course coincides with the sea-surface. 

 But whether the sea-surface as we proceed out to sea carries on 

 the same spheroidal form, geodesy does not enable us to deter- 

 mine. Perhaps accurate and extensive coast-surveys, as nearly 

 north and south as possible, would be serviceable for this end. 

 If the sea-surface and the inland sea-level can be shown to belong 

 to one spheroid, then the earth's mass is undoubtedly arranged 

 according to the fluid law. 



In our ignorance of the form of the ocean by direct measure- 

 ment, I think the close accordance of the ellipticity obtained by 

 pendulum experiments, many of them made at insular stations, 

 with that found by geodetic measures, is a fact of the last im- 

 portance, and in great measure compensates for the want of 

 direct measurement : and in our ignorance from direct sources 

 whether or not the spheroidal surface of the continents obtained 

 by geodesy is one of equilibrium, I think that the near agreement 

 of the ellipticity of that surface with the ellipticity obtained from 

 the fluid theory and an assumed law of density is a strong argu- 

 ment in favour of its being so. 



Gosulpoor, D. B., 

 June 11, 1866. 



XIX. Researches on the Mineralogy of South America. 

 By David Forbes, F.R.S., §■<?.* 



IV. 

 T\OMEYKITE. — This mineral, originally discovered in Chilef 

 by Professor Domeyko (after whom it has been named), 

 and there found in the copper-veins which traverse the 'strata 

 pertaining to the upper oolitic period, has now been met with 

 by the author at Corocovo in Northern Bolivia, occurring in the 

 beds of cupriferous sandstone of Permian or Triassic age, which 



* Communicated by the Author. 



f Has since been found in the copper-mines of the Lahe Superior district 

 in North America. 



