Page:The Journal of geology (IA journalofgeology21894univers).pdf/433
the Mendon and Mount Holly series and their associated phenomena, will now be considered.
Lithological differences.—These are many, and furnish important 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 prevailingly 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, crumbling often in the hand. Under the microscope, the cause for this is readily seen in the universal granulation that the rocks have suffered, a phenomenon 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