140 BULLETIN OF THE ESSEX INSTITUTE. 



tious ; but that the dikes are strictly limited to the granite 

 no student of this area will question. The Potsdam beds, 

 on the other hand, resting as they do directly upon the 

 granite, are in an extremely ffivorable position, strati- 

 graphic and otherwise, for filling fissures in the granite. 



In the foregoing descriptions of the dikes I have several 

 times stated that the rock resembles the Potsdam sand- 

 stone, and this resemblance is frequently indicated in my 

 note-book. In fact, I became convinced before the field 

 work was finished that the two formations are lithologi- 

 cally identical, except that the dike rock is usually 

 unstratified and rarely glauconitic. The glauconite, how- 

 ever, characterizes only a small part of the Potsdam 

 strata, and is often wanting altogether. This agreement 

 holds for both the Manitou and Manitou Park basins. 

 From the sandstone dikes north of Green Mountain Falls 

 I crossed the Manitou Park Basin to the highly inclined 

 Silurian and Potsdam outcrops along its eastern edge, and 

 found that a part of the latter formation is practically 

 indistinguishable from the dike rock. Cross describes 

 the sandstone of the dikes as of fine and uniform texture ; 

 but this is also the prevailing character of the Potsdam 

 beds, and my observations show that the dikes, as a 

 whole, embrace much coarse sandstone and some con- 

 glomerate, and that their range in texture is fully equal 

 to that of the Potsdam. Furthermore, the structural 

 contrast does not hold universally, for at several points, 

 as I have noted, and including also a part of the great 

 dike north of Green Mountain Falls, the dike sand- 

 stone is plainly stratified. The fticts that no sandstone 

 dikes have been observed in the Potsdam beds, and that, 

 so far as known, the dike rock is iiever calcareous and 

 rarely glauconitic, suggests its derivation from the lower 

 rather than the calcareous and glauconitic upper layers of 



