ALGONKIAN ROCKS IN VERMONT. 417 



the Mendon and Mount Holly series and their associated phenom- 

 ena, will now be considered. 



EVIDENCE OF DISCORDANCE BETWEEN THE MOUNT HOLLY 

 AND MENDON SERIES. 



Lithological differences. — These are many, and furnish im- 

 portant data for the classification of the two series into two 

 divisions. A hasty description has already been given of the 

 upper series and a still more imperfect one of the series below, 

 which, owing to its vast variety of rock phase, hardly warrants a 

 detailed description of each rock. In a large way it may be said 

 that the upper series is prevailingly schistose ; the lower prevail- 

 ingly gneissic. The rocks of the upper series can all be referred 

 indisputably to a sedimentary origin ; part, at least, of the 

 lower are of igneous origin, and a still larger part afford no 

 criteria which will enable us to assert their origin. Coarsely 

 crystalline limestones occurring in the core have in no case been 

 detected in the upper rock, and pebbly limestones or quartzites 

 are never met with in the Mount Holly series. Along the western 

 border of the range, from Sunderland to Chittenden, none of 

 the core rocks are seen interstratified with the Mendon series. 

 An association sometimes occurs, but only when there is evidence 

 for a faulted relationship . In the amphitheatre, where the lowest 

 rocks occur, none of the upper series have been found. Farther 

 north the lower terrane makes up but a small part of the surface 

 rocks ; the Mendon series capping all the prominent mountains as 

 far north as Nickwacket Peak. The chaotic occurrence and lack 

 of discoverable sequence in the core rocks find no parallel in the 

 relatively persistent and orderly arrangement of the upper series. 

 To the eye the core rocks have an older look ; they are commonly 

 loose-textured when weathered, cnmibling often in the hand. 

 Under the microscope, the cause for this is readily seen in the 

 universal granulation that the rocks have suffered, a phenome- 

 non strongly in contrast to the more coherent, less-sugared rocks 

 of the border. Other differences in the two series are found in 

 their mineralogical composition as a whole. Such differences 



