154 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



in the limestone, have a constant content of chroininm and nickel. I think 

 we may therefore assume that these elements have been concentrated from 

 extremely dilute solutions in and by the limestone, being derived perhaps 

 from the olivine rocks, and that they do not necessarily indicate that all the 

 minerals which now contain them were of eruptive origin. 



As to the other point, I find it very difficult to establish any certain 

 criteria to distinguish amphibolites derived from the complete alteration of 

 basic eruptives, or their tuffs, and those derived from argillaceous limestones, 

 and T have illustrated the difficulty in the series of figures on Pis. V and VI. 

 Where all residual structures are obliterated two rocks may become indis- 

 tinguishable. It is a question mainly of the actinolite network and the limpid 

 and untwinned albite mosaic of the amphibolite. The actinolite may have 

 either origin, and every stage may be observed from tremolite-schists cer- 

 tainly metamoi-jihic to actinolite schists. In several cases I have found 

 the albite mosaic closely like the untwinned albite growth of the adjacent 

 albitic schists, for which no one would suggest a connection with eruptives; 

 and that such a mosaic may readily form also from the action of heated 

 solutions, on a diabase is shown by its occurrence in the red Triassic trap, 

 described in Chapter XIII. It is rather a question of the easy formation 

 and solubility of albite or other plagioclase than of any necessary connec- 

 tion witli a basic eruptive. 



7. Formation of the Chester emery. — The Chester emery may have been 

 formed by the replacement of a portion of the limestone bed. Just as great 

 beds of limonite with gibbsite and allophane have formed, by replacement, 

 at the surface of the Stockbridge limestone, in Berkshire County, and of the 

 Bernardston limestone described below, so the formation of such a bed at 

 the surface of the former Chester limestone would explain the magnetite- 

 emery bed which now caps the Chester am})hibolite. 



Metamorphic agencies have changed the limonite into magnetite, while 

 the livilrated alumina compounds have become corundum, and continued 

 alteration has, as is usual with corundum, produced many interesting 

 minerals. 



The tourmaline can not be taken as a fumarole mineral of the olivine 

 rock, since, as it surrounds the emery lied in great quantity, and is not found 

 in association with the olivine rocks, it would tend to prove the emery bed 



