FORMATION THE BASIS FOR GEOLOGIC MAPPING 715 



various phases of the Hudson formation by the term " Hudson 

 shale," " Hudson slate," " Hudson schist." This system of 

 nomenclature, while recognizing the essential stratigrapic equiva- 

 lence and continuity of the various phases, allows these phases 

 to be discriminated from each other on lithologic grounds. On 

 maps this discrimination can best be shown by the use of some 

 of the various overprints used to denote metamorphism. 



Case 2. — The second case noted by Mrs. Willis is that of a 

 *' shale grading into a limestone with prolonged overlap, so that 

 the two rocks must be discriminated in one area. Not only 

 are they lithologically different but they have different strati- 

 graphic associations, and they should receive distinct " forma- 

 tional names. As noted under the preceding case, this is the 

 condition which usually obtains where there is a horizontal 

 gradation in character of sedimentation. The writer believes 

 that Mr. Willis has here suggested the proper treatment of the 

 question, but wishes to point out an actual case of some areal 

 importance in which this ruling has not been followed. 

 ' - The example occurs in the northwestern part of Massa- 

 chusetts. To the west of Hoosac Mountain, the Cambrian 

 quartzite is overlaid by Stockbridge limestone and this in turn 

 by Hudson (Berkshire) schist; on and east of the mountain the 

 limestone does not appear. It has been generally assumed that 

 the schist of the mountain represented both the Berkshire schist 

 and the Stockbridge limestone, and the name "Hoosac schist" 

 has been applied to it in several publications. 



What the writer wishes to point out is that, under the pro- 

 posed rules, the name at present used for one of those forma- 

 tions would seem to be untenable. For, if the eastern (Hoosac) 

 schist represents both the Berkshire schist and Stockbridge lime- 

 stone, the case becomes the same as 2, and in that case all 

 the schist should be called Berkshire and all the limestone 

 Stockbridge. If the transition be considered sudden, then it 

 exhibits a slight modification of case 3, and still all the schist 

 is Berkshire schist and all the limestone Stockbridge limestone. 

 If the relations at Hoosac Mountain be referable to overlap, 



