Chapter VIII. 

 THE BASIC AND ULTRABASIC INTRUSIVE ROCKS OF AMERICA. 



EASTERN NORTH AMERICA. 



Pre-Cambrian. — The numerous brilliant investigations of the ancient rocks adjacent to 

 the Great Lakes permit us to continue our studies of the development of basic rocks back into 

 the earliest periods of geological history. At the same time they are so extensive and varied 

 that a brief review must be very inadequate. The Keewatin lavas, the oldest igneous rocks 

 known, are a series of basaltic and andesitic rocks with some interbedded acid flows and show 

 when least metamorphosed marked pillow-structure. They are associated with tuffs and 

 agglomerates, and also with diabase-sills and locally (Rainy Lake, Lawson '13) are invaded 

 by gabbro and anorthosite, and in one region (Lake Abitibi, Wilson '13) by diorite and quartz- 

 porphyry also. They are generally of normal basaltic composition, and are more or less deeply 

 altered, passing into amphibolites and chlorite-schist. The "plagioclase," though originally 

 basic, has not infrequently been changed to albite by pneumatolytic action (Van Hise and 

 Leith '11, Wilson '13). They are associated with fine-grained sediments of considerable thick- 

 ness, indicating, perhaps, deposition during a geosynclinal depression. This was followed by 

 a period of intense folding, accompanied by the formation of huge batholiths, bosses, stocks, 

 and dikes of acid chemical composition, with minor amount of basic rock and peridotite at 

 Marquette (Van Hise '97). The movement may have been universal, or, as Blackwelder 

 suggests ('14), comprised movement at several periods. This movement was the Laurentian 

 revolution. 



After extensive planation the deltaic Sudburian sediments were laid down upon the 

 eroded surface of these ancient rocks (Barrell '15) and with them were formed a small series 

 of contemporaneous basic pillow-lavas and pyroclastic rocks, with a minor amount of rhyolite. 

 The majority of the analyses of these recorded by Clements ('99) show them to be of normal 

 composition, but in the case of two of the more felspathic rocks the small amount of lime and 

 relatively high percentage of soda suggests that some albitization and removal of lime has 

 taken place. This period of sedimentation and outpouring of lavas was followed by a second 

 period of crust-folding (the Algoman revolution), accompanied by immense plutonic intrusions 

 chiefly of more or less gneissic granites. In Wisconsin, however, Weidmann ('07) recognizes 

 as probably belonging to this period intrusions of gabbros, oeridotites, and diorite, followed 

 by granite and nepheline-syenite. 



After a further planation, Algonkian sedimentation followed, mostly deltaic, with suc- 

 ceeding widespread marine depositions in shallow basins, associated with extensive eruptions 

 of basic intrusive and extrusive rocks with breccias, the associated post-volcanic activity giving 

 rise to the immense iron-ore deposits of Michigan. Approximately coeval with these, there 

 were a similar series of eruptions near Hudson's Bay (Leith '10). After slight elevation and 

 erosion shales, sandstone, and conglomerates were deposited subaerially, associated with an 

 increasingly abundant series of lava-flows, the Keweenawan series, chiefly basalt or diabase 

 with subordinate amounts of rhyolite and volcanic ash, all quite normally subalkaline in com- 

 position. These were probably derived from fissure-eruptions. Faulting and some folding 

 occurred during the eruption of these lavas, for the lower portions are more steeply folded 

 than the upper. 



Among these were injected the immense Duluth mass of plutonic rocks, 100 miles in 

 diameter, which has invaded beyond the Keweenawan lavas into the Archaean complex. "It 

 52 



