^^2 PRINCIPLES OF STRATIGRAPHY 



set of changes producing meta-graywackes, meta-quartzite, etc. 

 Here the rock was first metamorphosed to a graywacke or a quartz- 

 ite, after which a second set of aUerations occurred. 



Apo is used as a prefix for rocks in which rnetasomatic changes 

 have taken place without entire loss of original texture or structure. 

 Thus a devitrified rhyolite is an apo-rhyolite. (Bascom-5.) This 

 name applies especially to rocks which have undergone diagenetic 

 alterations without loss of structure, though the chemical and min- 

 eral composition may differ greatly from those of the original rock. 

 The term is useful to call attention to the original rock from which 

 the new rock is derived. Thus quartzite is an apo-arenyte, gray- 

 wacke an apo-grit, etc. When new structures are produced we 

 ohtain slates, schists or gneisses. 



Variation in Mctamorphism of Strata. 



In a given series of metamorphosed strata a change may often 

 be noted in the intensity of the metamorphism as one passes from 

 point to point. This change may be along the strike of the strata 

 or across it. In the first case, it is generally gradual, the pro- 

 foundly metamorphosed strata of one region passing gradually into 

 the slightly metamorphosed equivalent of another. Thus the three 

 unconformable series of strata, the Archaean, the Lower Huronian 

 and the Upper Huronian, are so closely mashed and altered in the 

 western part of the I\larquette district of Michigan as to appear 

 completely conformable and suggest an inseparable series. In trac- 

 ing the formations to the less metamorphosed central and eastern 

 parts of the district, the three unconformable series are readily 

 recognizable as well as the original character of the formations. 



The change in the degree of metamorphism across the strike is 

 generally more abrupt. "Thus the rocks at the crown of an arch 

 or at the bottom of a trough may be only partly metamorphosed, 

 while the same formations on the limbs of the folds may be pro- 

 foundly metamorphosed, this being due to the difference in the 

 amount of shearing in different parts of the folds, or to the dying 

 out or change in character of orogenic movements across the 

 strike." (Van Hise-33 : 5P(?-(5oo. ) This change is well illustrated 

 in the Hudson River series, which in the Hudson Valley is prac- 

 tically unaltered, but eastward across the strike in the Taconic 

 range becomes schists and gneisses, while in Vermont and parts of 

 eastern New York the series changes to roofing slates. 



