Page:Popular Science Monthly Volume 10.djvu/282

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268
THE POPULAR SCIENCE MONTHLY.

New Hampshire.[1] The rocks have been carefully studied stratigraphically and lithologically, so that their place in the column is well understood, and the fossil is so allied to the Eozoön as to abundantly confirm all that has been held for it by its warmest advocates.

As a matter of convenience Mr. Hawes proposes to call the group of rocks affording these organisms greenstones, in allusion to their color. They have not been melted like a certain class of traps once called by this name, but have been metamorphosed somewhat; they embrace most of the chloritic and talcose schists, or, technically, "all basic metamorphic rocks whose predominant coloring ingredient is either hornblende, pyroxene, or chlorite." Those of special interest to us now are varieties of diorite and diabase, the first consisting mainly of hornblende and feldspar, the second adding labradorite to the constituents of the first-named rock. These rocks by many authors are regarded as of igneous origin.

PSM V10 D282 Protozoan fossil from conn lake n h.jpg
Fig. 7.—Protozoan Fossil, probably Stromatopora, from Connecticut Lake, N.H.

The method of examination employed in determining the composition of these greenstones is of some interest. A bit of the specimen is carefully ground to the thinnest dimensions possible, so that it can be examined optically under the microscope. With common and polarized light it is possible to understand the nature of the minutest minerals present, as well as the cavities contained in them. The study of rocks in this way has been prosecuted so energetically of late, that it is common to speak of the sub-sciences micro-lithology, micro-petrology, etc., and the appearances of every mineral are now well understood by those skilled in observation, so that the conclusions are often more reliable than those obtained by ultimate chemical analysis. Mr. Hawes combines in his studies the use of the microscope and chem-

  1. American Journal of Science, iii., vol. xii., p. 134.