SECOND HYPOTHESIS OF DERIVATION 



455 



Minerals. 



Weight of 

 paper-sec- 

 t i n s in 

 grams. 



Approx- 

 imate 

 specific 

 gravity. 



Product. 



Percentage 



by 



weight. 



Light colored 



.9485 

 1.2747 



2.65 

 3.50 



2.5135 

 4.4614 



29.09 



Dark colored 



70.91 





2.2232 



2.83 



6.2916 



100.00 



From the amount of P2O5, 0.23, in the chemical analysis, that of apatite 

 was calculated, 0.55, and deducted from the percentage of the light-col- 

 ored minerals, leaving 28.54 per cent for quartz. This mineral was 

 found, in a thin cross-section of the rock, to consist of a mass of crushed 

 granules. A little anorthite feldspar had been recognized by Emerson in 

 porphyroidal spots, but was not present in my specimen. The amount 

 of TiOg, 0.50, was divided, to calculate the small proportions of rutile, 

 0.25, and of titanite, 0.62, detected by Emerson. From the amount of 

 K2O. 0.19, the percentage of biotite was calculated, 1.76, using the pub- 

 lished analysis from Monroe, Connecticut. The aggregate of these three 

 minerals was deducted from the above percentage of dark-colored min- 

 erals, 70.91, to obtain that for hornblende, 68.28. The disregard of the 

 small amount of anorthite has tended to reduce somewhat the percent- 

 age of SiOj in the result (38,39) and so to increase the Alfl.^, whose esti- 

 mated amount, 23.41, doubtless exceeds the truth. The large estimated 

 amount of H2O, 4.48, probably signifies a state of hydration in the horn- 

 blende likely to be found in such a specimen of weathered amphibolite. 

 This also tends to minimize the percentage of SiOg. Only a small part 

 of the H2O (loss by ignition) is included in calculation of the molecular 

 ratio. 



The theoretical composition deduced (XIX, page 456) differs from 

 that of a hornblende of igneous origin mainly in its smaller content of 

 iron oxides. For comparison, the analysis (XX) is appended on a 

 hornblende derived from granular limestone, associated with tremolite, 

 titanite, chondrodite, and diopside.* It is apparent that hornblendes 

 of this origin vary widely in composition, and that the question cannot 

 be settled on chemical grounds. 



Third Hypothesis : Metamorphism of basic igneous Intrusions 



PRE VIO us IN VES TIG A TIONS 



The inclusion of these dark schists, together with the serpentine, in 

 the " Trappean division " of Mather, in his early report on this region, 

 was a natural conclusion from the ophiions prevailing at that time, par- 



