PREFACE. 



The following' memoir is the result of the fieldwork of the Archean 

 Division of the U. S. Geological Survey in northwestern Massachusetts, 

 (hiring the years 188.5, 1886, and 1887. 



The conclusions put forth were all arrived at before 1888, but the 

 publication of them was delayed until they should be either confirmed or 

 corrected by the results of further study in southwestern Massachusetts 

 and in central Vermont. 



The progress of our survey of western New England has fully con- 

 finned our interpretation of the facts observed in the Hoosac mountain and 

 Grreylock area. It has been our intention to keep wholly clear of the 

 Taconic controversy, and to confine our efforts to accurate study and inter- 

 pretation of structure. In the first part I have given a statement of the 

 sequence and bearing of the results and have advanced some theoretical 

 views in explanation of the sudden disappearance of the Lower Silurian 

 limestone against the western base of the Green mountain anticline. I 

 have also advanced a hypothesis, supported by observation in the northern 

 and southern Appalachians, to explain (through the presence of a previously 

 deeply disintegrated laud surface) the apparent conformable transition 

 between Archean or pre-Cambrian gneisses and Cambrian quartzite. This 

 almost insuperable difficulty is met with in many of the great crystalline 

 areas of the world, in passing from Archean or eruptive masses to the clastic 

 crystalline schists. 



The second part treats of Hoosac mountain — the central or crystalline 

 range of the Green mountains. The field work was performed by Dr. J. 

 E. Wolff, Mr. B. T. Putnam, and myself. The analysis of the results, the 

 petrographic study, and the presentation are by Dr. Wolff. Mr. Putnam 



