THE GRANBY TUFF BED. 479 



Farther south tiue fresh exposures can be studied where the road and 

 raih-oad come nearest too-ether, and here the round blocks are a foot in 

 diameter. All alonj^- the distance we have traced, the tuff is in great thick- 

 ness, and occupies the whole distance between the posterior trap rang-e and 

 the terrace sands adjoining the river, its varyino- width of outcrop depend- 

 ino- upon the heiglit to which it overhangs the trap. It is well exposed by 

 the cuttings of the railroad. 



The first reefs of sandstone containing foot-tracks appearing in the river 

 are brought up l)y a fault running N. 25° E. (see p. 473), which cuts off 

 the tuff, and south of this line where it appears in the quarry by the rail- 

 road; near the Holyoke line the tuff has dwindled quite suddenly to three 

 beds with a thickness of less than 3 feet each, included in the deep-red, 

 fine sandstone, and lying 3 to 4 feet apart, the lowest layer 10 feet above 

 tlie irregular surface of the trap, that thickness of sandstone having suddenly 

 intervened. 



President Hitchcock's descriptions of the tuff beds are full and clear.^ 

 The several repetitions of the bed given by him south of Mount Holyoke 

 are due to faulting. The occurrence in West Springfield and the occur- 

 rences mentioned at the base of the main trap and on its surface, and the 

 varieties described as " masses of red and gray sandstone embedded in a 

 seoriaceous paste," are separately discussed on pages 453-4(]0 and 476 as 

 cases of the inclusion of sedimentary material at the surface or base of the 

 flowing sheets of trap. 



THE ISOLATED MASS OF TUFF NORTH OF THE SEVENTH CORE. 



A half mile north of the seventh core, described in the next section 

 (p. 482), and a little more than a mile north of J. McGrath's house and 

 reached by a wood road from this house, in the deep woods in the north part 

 of Granby, is a great isolated mass of the coarse rusty diabase-sandstone, 

 which stands perhaps 12 feet high and rises like a great telescope upon a 

 massive pedestal. The mass does not seem to be more than a rod square 

 beneath the surface, and must hars-e been dropped into the sandstones here 

 by a fault, of which in the covered and heavily wooded region no other 

 trace can be foimd. It shows that the tuff extended half a mile north of its 

 present outcrop. 



'1844, Explanation of Geological Map; 1848, Am. Jour. Sci., 2d series, Vol. IV, p. 199. 



