ACADEMY OF SCTENCES] AMERICA. 59 



since the gentle folding of these rocks can be traced with increasing intensity into the Appa- 

 lachian folds, the small amount of shearing visible between the various beds of sandstone must 

 have occurred during the Appalachian folding, and concludes that as the ultrabasic dikes 

 have been moved by this shearing they must be of pre-Permian age. Kemp and Ross ('07) 

 comment on the strongly marked contact-effect of a dike of this rock which invades the coal 

 measures of southwestern Pennsylvania, which is in accord with Holland's observation ('95) 

 in India. In Crittenden County, Kentucky there is a large dike of this rock more than 20 feet 

 wide. (Most of the dikes range from a yard to less than an inch in width.) This dike occupies 

 the plane of a fault which throws Carboniferous rocks a distance of 800 feet. In Elliott County, 

 a narrow dike of a similar petrographical nature invades almost horizontal coal-measures and 

 contains fragments of shale. Diller ('87) remarks that " the very slight disturbance suffered by 

 the strata through which the peridotite reached the surface suggests that the extrusion may not 

 have been connected with great orographic movements at the close of the Carboniferous period, 

 but rather with subsequent dislocations * * * which occurred at a much later date. " 



In Pike County, Arkansas, similar peridotite occurs forming a small stock 2,000 by 1,600 

 yards in dimensions. It was originally described by Branner and Brackett ('89), whose report 

 is cited in full by J. F. Williams ('90). The stock lies within Carboniferous formations, but ad- 

 jacent to it is a dike of peridotite invading the Lower Cretaceous sandstone, which lies horizon- 

 tally, and this dike contains abundant inclusions of Paleozoic and Mesozoic rocks. Branner 

 suggested that the subsidence in Tertiary times is in some way connected with the intrusion of 

 these rocks, while Williams adds that it is evident that the time of their intrusion was not far 

 removed from that of syenitic and monchiquitic rocks of other parts of the State. Purdue ('08) 

 has found other pipes of the peridotite near Murfreesboro, in Arkansas, which, like the South 

 African rocks of this character, are associated with diamonds. Recent observations by Glenn 

 ('12) lead to the belief that the period of intrusion occurred between Lower and Upper Creta- 

 ceous times. 



Thus the rocks of this group throughout the United States form narrow dikes or small 

 stocks in horizontal or slightly folded, though not unfaulted, sediments, a mode of occurrence 

 utterly distinct from that of normal gabbros and peridotites. We have seen that similar asso- 

 ciations hold in India; they also hold in South Africa. For this reason Sacco's ('05) classing 

 of these rocks with the normal ophiolitic group can not be supported. 



Probably consanguinous with these are the series of igneous rocks forming the alkaline 

 complex of Magnet Cove. Washington ('00) interprets this as a thick complex laccolite, but 

 Dr. Harker ('02) suggests that it may consist of two thin laccolites bulged quaquaversally sub- 

 sequent to their consolidation. The rocks are very varied, foyaite, and such basic and ultrabasic 

 types as ijolite and jacuipirangite being present. They were injected without notable folding 

 in late Cretaceous times, for the bulk of the Cretaceous strata in this region are horizontal. 



We may here mention two other alkaline complexes: The Monteregian Hills near Quebec 

 consist of essexite and nepheline-syenite with a series of diverse alkaline dike-rocks. Adams 

 ('03) considers they are laccolitic and have been thrust through horizontal Devonian rock. 



In North Greenland Steenstrup ('84) describes a sill of picrite-porphyry 120 feet thick in- 

 vading the Kome beds which are flat-lying "Miocene" or Cretaceous sediments. It consists 

 almost entirely of olivine, lying in a yellow or pale-green clear base. These last three occur- 

 rences would be appropriately classed with our " alkaline-plateau " group of intrusions. Possibly 

 the occurrence of ultrabasic rock in Greenland figured by Daly ('14, p. 447) should be included 

 here. A comprehensive illustration of these rocks, also due to Arnold Heim, is that given in 

 La Face de la Terre (Tome iii-4, 1918, p. 1525) but the literature concerning this and other 

 interesting complexes in Greenland is not accessible to the writer. In particular may be men- 

 tioned the niagnesiari rocks of Ellesmere Land described by Bugge ('10). 

 81795°— 26 5 



