The feldspathic rock with which the titanium deposits of Virginia are associated appears to be an anorthosite, although the plagioclase is somewhat more sodic than in normal anorthosites. That is, this rock shows evidence of having been introduced as a mush‐like mass of crystals and intersititial magma, and of undergoing very thorough crushing during introduction. This rook was later very thoroughly altered by hydrothermal solutions which migrated through the fractures in the feldspar rook and new minerals including the titanium minerals—rutile and ilmenite—were introduced. The elements introduced into the feldspathic rock ware TiO 2 FeO, MgO, P 2 O 5 , and F. The other elements that went into the formation of new minerals were derived from the feldspar originally present. There was a distinct order in the abstraction of the introduced elements from the carrying solutions, as shown by the zonal relatione of the secondary minerals. Ferrous iron was abstracted first in the formation of the ferromagnesian silicates and ilmenite. Magnesium and titanium, in the absence of iron, traveled farther from the source of the solutions and deposited rutile and magnesian silicates. At greater distances most of the introduced elements had been abstracted from the solutions but notwithstanding this the feldspar was extensively altered, but without any marked changes between the chemical composition of the original feldspar rook and the resulting altered rook.
Specimens of glass resulting from the explosion of the experimental atomic bomb at Alamogordo, New Mexico, have, by various means' come into the hands of numerous institutions and collectors. Notwithstanding this availability, no studies of the material have been published, although a mere description of its physical character would have no bearing on atomic problems. Samples of the glass were collected by C..A. Anderson of the Geological Survey with the permission of Major General Leslie R. Groves, Commanding General for the Manhattan District. The results of a brief study of this glass, made in the laboratories of the U. S. Geological Survey, will no doubt be of interest to mineralogists and petrologists. The glass, in general, formed alayer 1 to 2 centimeters thick, with the upper surface marked by a very thin sprinkling of dust which fell upon it while it was still molten. At the bottom is a thicker fiIm of partly fused material, which grades into the soil from which it was derived. The color of the glass is a pale bottle green, and the material is extremely vesicular' with the size of the bubbles ranging to nearly the full thickness of the specimen. Figure 1 shows photomicrographs of typical specimens of the
As altered volcanic material are being found in the Paleozoic, there is an interest in the criteria for their recognition. The relict glass structure, the nature and habit of the crystal grains, and the physical properties present evidence that indicates the mode of origin. Bentonite is a common type of altered volcanic material and may be recognized by the volcanic structure preserved in the clay substance, and the origin is confirmed by the presence of volcanic minerals. The chemical and physical properties of bentonite are variable and many are indefinite; thus, although they may contribute to its recognition, as a rule they can not be relied upon completely. Bentonites from several of the eastern United States, Nova Scotia, Arkansas, Texas, and New Mexico are describe in detail. It is concluded that many volcanic materials, even though highly altered, may be recognized by microscopic and physical studies and may contribute to the interpretation of the geologic history of a region.
Four exposures of peridot{re occur near Murfreesboro, Pike County, Arkansas, and.three of these have produced diamonds.The exposure first discovered lies 2• miles south-southeast of Murfreesboro, near the confluence of Prairie Creek with Little Missouri River, and is thus named the Pr;firie Creek peridotite area.(See Fig. 58.)Diamonds were discovered on the area in I9O6 and since then several thousand stones, ranging in weight from a small fraction of a carat to 2o• carats, have been pro-
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