ON DHE Nato OF IGNEOUS - INTRUSIONS: 
In an article in the preceding number of this JouRNAL, certain 
igneous intrusions in the vicinity of the Black Hills of Dakota, 
termed plutonic plugs, were described, and the similarity pointed 
out between the domes of sedimentary beds raised above them 
and the far greater dome from which the Black Hills have been 
sculptured. Other mountains of the Black Hills type in Wyo- 
ming and Colorado were also mentioned. In the present article 
I wish to direct attention to the fact that the dome-shaped uplifts 
referred to, form part of a genetically related series of disturb- 
ances caused by subterranean intrusions of igneous rock. Vari- 
ous stages in this series are shown by intruded sheets, laccolites 
plutonic plugs, and great dome-shaped uplifts of which the Black 
Hills furnish the type. 
Intruded sheets —As is well known, igneous rocks frequently 
occur in widely spread sheets, included between but litttle dis- 
turbed sedimentary strata. Although the enclosing sedimentary 
layers are frequently without conspicuous signs of having been 
disturbed, metamorphism of the sedimentary beds resting on 
the igneous rock at the surface of contact, furnishes proof that 
the latter was intruded in a molten condition between planes of 
bedding, after the sedimentary layers were consolidated. An 
example of an intruded sheet is furnished by the Palisade trap 
of New Jersey and New York. Including what is reasonably 
supposed to be a portion of this sheet, but which is separated 
from the main exposure by a covering of Cretaceous clays, its 
length measured along the curve formed by its outcrop is 90 
miles. A straight line joining the most northern and most south- 
ern exposures is 70 miles in length. The sheet varies in thick- 
ness from about 300 feet at Jersey City, to at least 850 feet at 
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