. 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. st direction was impeded by the initial dip encountered,which compelled the intrusive to phuige downward if it would remainbetween the same stratigraphic planes. In other words, when the21 OEOL, PT 3—01 15 210 THE LACCOLITHS OF THE BLACK HILLS. steeper northeast dips were encountered, the force of wedging aparthorizontal rocks was conAerted into a thrust against upturned result was more violent deformation of the initially dip
. 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. st direction was impeded by the initial dip encountered,which compelled the intrusive to phuige downward if it would remainbetween the same stratigraphic planes. In other words, when the21 OEOL, PT 3—01 15 210 THE LACCOLITHS OF THE BLACK HILLS. steeper northeast dips were encountered, the force of wedging aparthorizontal rocks was conAerted into a thrust against upturned result was more violent deformation of the initially dipping bedsto form an unsynmietrical laccolith with steep northeasterly dip. InChapter V it is shown liy experiment that the fractures of a dome areradial on the crest, gaping upward and closed below, they are concen-tric on the periphery, gaping downward. These peripheral strike frac-tures open toward the poi-phyry core. If the dome is imsymmetrical,the rupture occurs tirst on the side of most intense flexure. In DomeMountain, after the limit of flexure Avas reached on the northeast, thebeds fractured and gaped open to admit the porphjay to higher hor-. izons. In Lost Gulch, on the eastern flanks of Dome Mountain, theupper contact of this laccolithic mass with Silurian limestone is wellshown. Here the limestone is seen dipping at an angle of 40^, soviolently strained and contorted at its contact with the porphyr}^ thatit is infolded in a small pinched syncline plunging northeast betweentwo lobes of the laccolith. A few hundred paces to the north theporphyry may be followed continuously, crossing Silurian, Carbon-iferous, and Minnelusa })eds; here the flexure has passed into phenomenon is the same as that represented by the cross-cuttingsill in fig. 77, the only difference being that in this case large lacco-
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