:iZABETHTOWN AND 



Endeavors to recast these analyses according to the mineralogy 

 exhibited by the thin sections have not been very satisfactory. 



No. I has involved several assumptions. The soda was first all 

 assigned to albite. The remaining alumina was used for orthoclase, 

 there being none for anorthite. The residue of potash, with the 

 necessary ferric iron, was used for biotite, affording a variety ab- 

 normally high in the alkali but making very little dift'erence in the 

 gross result. Everything else was assigned to hornblende, except 

 for the little magnetite and titanite. The slides reveal quartz, 

 microperthite, biotite, hornblende, magnetite and zircon. 



No. 2 presents no difficulties. The great preponderance of the 

 light colored minerals is striking, since about 95 per cent consist 

 of these. The remainder is chiefly augite although this mineral is 

 a very obscure component to the eye. The richness in the albite 

 molecule is striking. While this mineral is chiefly observed in 

 microperthite, it can not obviously be altogether in this form, srnce 

 13 per cent orthoclase could hardly contain 53 per cent albite. 

 While this rock is beheved to be an acidic differentiation product 

 from the syenitic magma, it is only fair to state that in one respect 

 the analysis is similar to the usual run of slates and argillaceous 

 sandstones, in that with high silica the magnesia exceeds the lime. 

 The opposite relation usually holds for the eruptive rocks. Yet 

 the amounts are small and other considerations lead to the interpre- 

 tation as an igneous variety. The presence of zircon in the acid 

 member and its failure in the basic is an interesting corroboration 

 of our ordinary conceptions of the home of this mineral. 



When the recasting of nos. 2, 3 and 4 are undertaken, difficulties 

 arise in providing so much soda with sufficient silica to satisfy the 

 albite molecule, at its ratio of one soda to six silica and yet have 

 sufficient silica left to form unisilicates which take up the other 

 bases. One can not but infer that the emerald-green pyroxene itself 

 carries soda, but it can not be "in the form of the acmite molecule, 

 NaoO.Fe20o4Si02 since we have so little FcoOg, while magnetite is a 

 certain component of the rock. The alumina is also in excess, and 

 can only be provided for by the spinel molecule. Yet no spinel has 

 been detected in the slides. Any form of the olivine molecule, and 

 any corundum which we would use according to the methods of 

 the '' Quantitative System of Classification of Igneous Rocks " are 

 also not to be observed in the slides. Nephelite can also not be 

 detected in the slides although with basic rocks such as these, the 

 rather high percentage of soda would seem to call for it. 



