impregnated precambrian strata 241 



Interpretation of Precambrian impregnated Strata 



The great fact which stands out from the results of this study, and 

 which we are now finding in practically all the later work, is the recog- 

 nition of one or more very ancient series of strata, as distinct from in- 

 trusive granite. Sometimes they are undoubtedly sedimentary and con- 

 tain limestones, quartzites, and thinly laminated gneisses or schists which 

 represent ancient shales. The Grenville of eastern Canada, of the Adi- 

 rondack^, of the Highlands of the Hudson and New Jersey, and probably 

 of southeastern Pennsylvania, Maryland, and Delaware, is a familiar 

 illustration. Here and there, not too greatly altered members are easily 

 recognized to be sediments; but with them are mixed rocks, the "migma- 

 tites" of the Scandinavians, which have first been considered excessively 

 metamorphosed sediments ; then extremely variable igneous rocks ; and 

 finally, and probably correctly, original sedimentary deposits penetrated, 

 soaked, and altered by the so-called "juice of the magma." The juice 

 carried with it into the surrounding rocks the elements of many silicates 

 and. penetrating especially along bedding planes or planes of schistosity, 

 introduced pegmatitic matter far and wide. 



These or similar changes did not entirely escape the keen observation 

 of early workers. All will recall the quaint diagrams in the early text- 

 books, which picture the ancient metamorphic strata resting on the 

 underlying granites, from which pegmatite and other dikes wriggle their 

 way upward and outward. Elie de Beaumont, Michel-Levy, and other 

 French observers mention frequently "agents mineralisateurs," or, as we 

 say, "mineralizers." 



In later years French observers began to speak of lit-par-lit injection — 

 a phrase which we especially owe to F. D. Adams for its introduction into 

 North American usage. I need only to remind you of the extraordinarily 

 suggestive monograph of Adams and Barlow 22 on the Haliburton and 

 Bancroft area of Ontario, and the interesting way in which the former 

 traced out the entrance of silica, probably with some bases, along the bed- 

 ding planes of limestones; the lit-par-lit production of feather amphib- 

 olites; and finally the reorganization of the limestone into amphibolite 

 itself. Adirondack workers became familiar with mixed rocks. In one 

 place we met gneissoid members of general syenitic composition, but with 

 blue labradorite augen, which must have come from neighboring anortho- 

 site. Elsewhere amphibolites with the same blue augen ; again, gneissoid 

 granitic members with long, thin streaks of amphibolite ranged in notably 

 parallel grouping; in still other exposures, bedlike masses of magnetite 

 folded like sediments, but in wall-rocks indistinguishable from familiar 



XVI— Bull. Geol. Soc. Am., Vol. 33, 1021 



