SECOND HYPOTHESIS OF DEKIVATIOX 451 



At the same time the scarcity of iron is a significant feature in these 

 intermixed silicates ; neither augite nor hornblende has ever been found 

 in these limestones, and very rarely biotite, in small quantity. Beds of 

 hornblende gneiss or schist occur in the gneisses not far from the lime- 

 stones, but never in contact with them nor passing into them. 



A curious association of granular limestones and dolomites with black 

 hornblende schists has been elsewhere observed — for example, in the 

 Grenville series of Canada and in parts of New York state, in Orange 

 county and in Essex county — the Adirondack region, where large out- 

 crops of limestone are " associated with black hornblendic schists, which 

 often cover a much greater area than the limestone, and which are so 

 characteristic that we have come to recognize them as an indication of 

 the series."* But it does not necessarily follow that in any of these 

 cases there must be a genetic relationship between the two rocks. The 

 black hornblendic and pyroxenic schists and gneiss in that region have 

 been attributed by the same observer to possible alteration of aluminous 

 shales or of gabbroic intrusions. 



COMPARISON WITH RESULTS IN MASSACHUSETTS 



For satisfactory study of any tract of highly crystalline schists of sus- 

 pected sedimentary origin we are fortunate in being able to refer to the 

 investigation of similar schists in that broad metamorphic terrane of un- 

 doubted sediments, in less altered condition, which occupies the western 

 part of Massachusetts. It comprises deposits of a great variety of con- 

 stitution and in all stages of alteration, belonging tothe Algonkian, Lower 

 Cambrian, Lower Silurian, and Upper Devonian periods. My own work 

 at Cummington, Goshen and Chesterfield in 1878 and during succeed- 

 ing summers, has inclined me to put great confidence in the cautious 

 conclusions deduced by Professor B. K. Emerson from his thorough re- 

 search toward the solution of this difficult problem. It will suffice for 

 my present purpose to refer brieja.y to some remarkable results. f The 

 incipient products of alteration of impure argillaceous limestones and 

 dolomites by the process above suggested have been found (including 

 some better represented in eastern Massachusetts and elsewhere) to be 

 the following: 



Stage 1. Tremolite limestone or dolomite. 

 Actiiiolite limestone or dolomite. 

 Hornblende limestone or dolomite. 

 Diopside (canaanite) limestone (or augi-calcite). 



* J. F. Kemp : Bull. N. Y. State Mus., vol. iii, 1895, pp. 329, 335, and Bull. Geol. Soc. Am., vol. 6, 

 1895, pp. 24-2, 246, 248, 251, 261. 



jGeology of Old Hampshire County, Mass., U. S. Geol. Survey, Men. xxix, 1898, pp. 147-155, 163. 

 282, 306. 



