142 BULLETIN OF THE ESSEX INSTITUTE. 



Rainbow Fall. On the south side of a small lateral gulch 

 a sharp, inverted flexure of the basal Potsdam beds can 

 be seen (Fig. 4). Throughout the flexure the sandstone 

 is beautifully slickensided in various directions precisely 

 as in the sandstone dikes. The extremity of the sharp 

 lower curve is not clearly exposed, but on both this and 

 the main curve above it the bedding is much obscured or 

 completely effkced, and the sandstone closely resembles 

 the dike rock. 



Many of the sandstone dikes are one hundred feet or 



- Red. 



Hg.^- 





4.+X .4-+, +-.-U-L^ + 





+ + ■*",--' ••••■.••••..■ •>: / "- " 



/ — 



-^^^' 



mor in oreadth, and the largest, as described by Cross, 

 five hundred to one thousand feet ; and certainly no single 

 featdre of the dikes is more sio^nificant than the 2XQsX 

 breadth of individual examples. Although presenting, 

 apparently, an insuperable obstacle to all the other sug- 

 gested explanations of the sandstone dikes, it offers no 

 difficulty whatever to the theory proposed here, for we 

 have only to make the extremely prol)able supposition 

 that sheets of granite of varying width and bordered by 



