FOLIATION IN THE PRE-CAMBRIAN OF NEW YORK 595 



the whole region, for any such pressure, great enough to produce 

 close folding, would have produced a high degree of parallelism 

 of strikes throughout the region. 



Local contortions. — Local contortions or sharp folds in the Gren- 

 ville strata are by no means uncommon, being especially prominent 

 in the limestones and closely associated hornblende and pyroxene 

 gneisses. Such plications have usually been regarded as strong 

 evidence for large-scale folding, being thought of as minor folds 

 superimposed upon large-scale folds. Now, in the first place, it is 

 the writer's experience that such local contortions or plications are 

 very largely confined to the limestone beds, which are easily the 

 most plastic of all Adirondack rocks. In the second place, the 

 crowding of a batholithic magma against the invaded Grenville 

 strata, or the catching of a mass of Grenville between two batho- 

 lithic magmas, would readily account for more or less local contor- 

 tions or even puckering of strata without any assumption of orogenic 

 or severe lateral pressure exerted throughout the region. The 

 shouldering action of the upwelling magmas must have produced 

 rather severe local pressures. Regarding the Glamorgan batholith 

 of Ontario, Adams and Barlow say that the Grenville rocks form- 

 ing the periphery on several sides, "being squeezed between this 

 and the adjacent batholiths, are too highly contorted .... to 

 display the prevailing dip distinctly." 1 Evidently such structures 

 do not necessarily call for severe regional compression. 



Summary. — To summarize, there is no known evidence within 

 the Adirondack region that the Grenville strata have ever been 

 highly folded or severely compressed, while many broad Grenville 

 belts are known to be only very moderately folded, and many 

 masses, large and small, are merely tilted or domed at various 

 angles. Very locally the strata are sometimes contorted or plicated. 

 The structural relations are therefore best explained as having been 

 the result of slow irregular upwelling of the more or less plastic 

 magmas, probably under very moderate compression, whereby 

 the Grenville strata, previously deformed very little or none at all, 

 were broken up, tilted, and lifted or domed. The stratification 

 surfaces of the Grenville were thus swung into general parallelism 



1 Adams and Barlow, Geol. Surv. Can., Mem 6, 1910, p. 15. 



