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ALGONKIAN ROCKS IN VERMONT.
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it with probability to an original diabase.[1] Remains of an original bisilicate (augite) can still be found in the rocks. Whether diabase or basalt their occurrence in sheets traversed by dikes of the same material and their great abundance lead me to consider them surface flows or intrusives. Their abundance may be cited as evidence of extrusive origin since it is extremely unlikely that any area, reasoning from analogy, would be traversed by so large a number of intrusives. This view is also sustained by the fact that diabases and basalts are prevailing surface flows. Such regions as the Triassic (Newark) of the eastern United States, Keweenaw Point, the western plateau, and the Deccan being examples. Their restriction to the Mount Holly series not only points to their extrusive origin, but whatever their origin they afford almost positive evidence of an unconformity at the top of the series; if intrusive, we should naturally expect to find them occurring in the Mendon series, which is not the case; if extrusive, their occurrence only in the core rocks is even more in favor of the proposed subdivision. As to the importance of the evidence afforded by these rocks no better confirmation can be found than the following from Van Hise.[2] "Eruptive rocks are often an important guide in determining structural discordances. These are valuable when the older series has passed through an epoch of eruptive activity before the newer series was deposited. In such cases, bosses, contemporaneous or intrusive beds, volcanic fragmental material or dikes may occur in the older series which nowhere are associated with the newer. It is possible, of course, that eruptives may penetrate the inferior members of a series and never reach the higher formations; but if it is found that the supposed inferior series is associated with abundant material of igneous origin which never passes beyond a certain line, it is almost demonstrative evidence of the later age of the newer series."

  1. ↑ Geology of the Green Mountains in Massachusetts, by R. Pumpelly, J. E. Wolff, T. Nelson Dale, and Bayard T. Putnam, Monograph U. S. Geol. Survey, Part 3, submitted in 1889.
  2. ↑ Correlation Papers—Archæan and Algonkian, Bull. No. 86, U. S. Geol. Survey, p. 520.
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