I20 MINERALOGY AND LITHOLOGY. 



microscopic sections it is scon to possess a concentric radial structure. 

 This structure is wholly microscopic, for, although chlorite is noted for 

 assuming fan shapes, no arrangement of the kind is macroscopically vis- 

 ible. This chlorite was evidently formed from the products of the de- 

 composition of the rock. 



So. ViRiDiTE, Delessite, Diahantite. 



When now we come to consider what is the chlorite that is formed in 

 the basic rocks themselves, the investigations are quite conflicting. It is 

 known that in almost all the old basic eruptive rocks, much chlorite has 

 been formed at the expense of the augitic and hornblendic constituents, 

 and the nature of this chlorite has been studied by many mineralogists. 

 It is well known that these are chlorites which contain much iron, and 

 are liable to give the most variable formulae on account of the ready oxi- 

 dation of this iron. Delessite is probably the name by which they are 

 best known, but Kengott would refer them all to the species last de- 

 scribed (the ripidolite of G. Rose). The formula of delessite could never 

 be fixed because the analyses were so various. Liebe came very near 

 it, and from the mean of several analyses deduced a formula, and named 

 the mineral diabantachronyn, because it was the coloring mineral of dia- 

 base. I obtained this chlorite in a very pure state from the diabase near 

 New Haven, and got an analysis which seemed to me must be pretty 

 near the original composition of this chlorite, since the iron was essen- 

 tially all protoxide. The analysis confirmed Liebe's results, and so I 

 took the name he had given it, only shortening it to diabantite. The 

 analysis was as follows : Si O 33.68, Al^Oj 10.84, Fe^O 2.86, Fe O 24.33, 

 CaO .73, MgO 1C.32, water 10.02.* The quantivalent ratio of its bases 

 and silica are as 1:1, while in the prochlorite, the analysis of which is 

 given, they are as 3 : 2, and the formula of diabantite, as drawn from the 

 analysis, is (Fe, Mg)'"' (A\\ Fe')' Si' H'« 0'\ The exact equivalence of 

 the elements, its undecomposed appearance, and all its characteristics, 

 indicate that this is very likely the chlorite of diabases, and that delessite 

 is likely the same thing in an impure condition. It may at least answer 

 the purpose of this study to assume that we know the approximate com- 

 position of the chlorite of diabase, and that it is certainly near that indi- 



* American Journal of Science, iii, vol. ix, p. 455. 



