188 PERIDOTITE. 



The vast majority of analyses show a percentage of alumina less than 5, 

 but the picrites tend to have more (mostly between 10 and 20), as also some 

 of the others, thus approaching the basalts. The highest percentage is in the 

 Muddor stone analyzed by Frank Crook — 26 per cent — a doubtful analysis. 



The amount of iron in various states distinctly varies in the meteoric and 

 terrestrial peridotites ; so much so, that from the sum of the percentages the 

 class can readily be inferred in most cases. In the meteorites the iron in 

 every condition varies somewhere about the mean of 30 per cent — those 

 forms having the lower percentages of silica, usually more than 30, and 

 those having the higher percentages of silica, contain generally less than 30 

 per cent. 



In the terrestrial peridotites the iron percentages are commonly below 10, 

 and universally below 20 per cent The more highly altered the rock, the 

 less is the percentage of iron. This agrees with the microscopic observations, 

 and shows that in the process of alteration much iron is removed and stored 

 up elsewhere. 



Lime is either absent in the peridotites, or less than 5 per cent in the vast 

 majority. The cases in which higher percentages are found are apparently 

 due to incorrect analyses, or, in the main, to the picrites and two serpentines 

 which are associated with gabbro, and may possibly be altered forms of it — 

 a conclusion to which their relatively high contents of alumina and lime 

 point. The picrites show their near relation to the basalts in the same way. 

 19.50 per cent of lime (the highest) was found in the typical Iherzolite from 

 Vicdessos, in an analysis published in 1813. It is thought that here the lime 

 and magnesia were poorly separated, or else the peridotite has been in some 

 way modified by the limestone through which it was erupted. No other 

 peridotite contains over 13.61 per cent of lime. 



The magnesia percentage is relatively high but variable, and it tends in 

 the meteorites and picrites to lie below 30 per cent, and in the other perido- 

 tites to be above that. Of course, the nearer the rock comes to being a pure 

 olivine one, or to be entirely altered to serpentine, the more nearly the 

 percentages correspond to the typical ones for those minerals. Part of the 

 variability in the percentage of magnesia is undoubtedly owing to poor 

 analyses. 



Chromium, nickel, and cobalt are quite commonly present, as are also 

 phosphorus and sulphur in the meteoric forms. Sometimes copper and tin 

 are found. The more highly altered the peridotites, the more apt is water 



