588 WILLIAM J. MILLER 



The metamorphic rocks of igneous origin, given in regular order 

 of geologic age, comprise the anorthosite series, the syenite-granite 

 series, and the gabbros, all of which show quite varied degrees of 

 metamorphism. All are intrusive rocks and younger than the 

 Grenville. Among the criteria for recognizing their igneous origin 

 are: preservation of original rock textures, such as the porphyritic 

 and the diabasic; relative homogeneity in large bodies; common 

 occurrence of distinct inclusions of older rocks; intrusive contacts, 

 often with dikes from the large bodies penetrating the older rocks ; 

 very common occurrence of zircon and zoisite in fresh, well- 

 crystallized grains. 



THE GRENVILLE AND ITS FOLIATION 

 FOLDING OF THE GRENVILLE 



Character of the Grenville series. — The Grenville series comprises 

 the oldest rocks of the Adirondack region, and they are, in fact. 

 among the most ancient known rocks of the earth's crust. They 

 consist of a great mass of thoroughly crystallized sediments, such 

 as limestones, sandstones, and shales which have been changed to 

 crystalline limestones, quartzites. and various gneisses and schists. 

 A more or less well-developed foliation is always parallel to the 

 stratiheation surfaces which are usually distinctly preserved in 

 spite of the crystallization. Granulation is not common. Gren- 

 ville strata are well represented throughout the Adirondack region, 

 their distribution being very irregular or "patchy" in small to large 

 areas. They are considerably less extensive than the later (intru- 

 sive) syenite-granite series, which latter, together with the Gren- 

 ville, makes up the great bulk of Adirondack rocks. Neither top 

 nor bottom of the Grenville series is known, though thicknesses 

 of from 10,000 to 20,000 feet are actually shown in single sections, 

 and the total thickness is doubtless much greater. Adams and 

 Barlow report a Grenville section about eighteen miles thick in 

 Ontario. The strata are often tilted at high angles or very moder- 

 ately folded, and sometimes locally contorted. There is a general 

 tendency toward a northeast-southwest strike of Grenville masses 

 in the Adirondacks, but there are many important exceptions. 



