. Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society. crjsts showing striation (prol)ahly oligoclase) and a decomposedbisilicate. Other cases of the rock with phenocrystic pink (microcline)feldspar cutting the rocks of the rhyolite-andesite series occur on thespur south of White Kock, just above Dead wood, and on the ridge westof the head of Spruce Gulch. Near the sawmill north of Kirk Hilla gray rock containing much mica and 3ellow feldspars occurs asa dike cutting light-gray rhyolite whi
. Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society. crjsts showing striation (prol)ahly oligoclase) and a decomposedbisilicate. Other cases of the rock with phenocrystic pink (microcline)feldspar cutting the rocks of the rhyolite-andesite series occur on thespur south of White Kock, just above Dead wood, and on the ridge westof the head of Spruce Gulch. Near the sawmill north of Kirk Hilla gray rock containing much mica and 3ellow feldspars occurs asa dike cutting light-gray rhyolite which shows some hornblende. Itis prol>able that these are mostly phonolitic rocks which cut rocksof the rhyolite-andesite magma. Irving has described four cases ofphonolites cutting (juailz-porphyries and diorite-porphyries. A very unusual case (fig. 02) is shown in a section near Aztec onthe Burlington Railroad (Spearfish branch). Here a mass of greenphonolitic porphyry is apparently cut by an irregular swelling intru-sion of white rhyolite of the fine-grained decomposed type so commonin the region. Both occur, as shown, at a sloping dike contact with. Fig. 61.— i>h()nolite out rimd, lui AGE OF PORPHYRIES. 185 Cambrian shales. All the evidence in this case would show the rhyo-lite to be the 3^ounger, or contemporaneous with the phonolite. Thesecond alternativ^e is the more probable. The two magmas may gradeinto each other, and in this case perhaps the green tegirine rock wasstill viscous when the stream of white rhyolite was injected throughit. It is hoped that more extended study of the petrography of theregion will throw light on these relations. (4EOLOGICAL, AGE OF PORPHYRIES. The geological age of the porphyries may be stated in only the mostgeneral terms. The Bear Butte and Little Missouri Buttes laccolithswere intruded into Benton Cretaceous and were unquestionably cov-ered by Niobrara limestone. Jenney has found pebbles of Black Hillsporphyries in t
Size: 2063px × 1211px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorunitedst, bookcentury1900, bookdecade1900, bookyear1901
