GEOLOGY OF THE NORTHERN ADIRONDACK REGION 311 



foliation in varying degree. In all of them the amount of 

 change varies from place to place. In a rude degree all the 

 intrusive masses show greater metamorphism toward their edges 

 than at their centers, and because of this the large masses are 

 apt to show a larger proportion of slightly metamorphosed 

 rock than the small masses do. All of them also show local 

 variations in this respect. As a general rule the anorthosite 

 is the least metamorphosed of all the intrusives. If it were 

 younger than the others, its condition in this respect would 

 find ready explanation; and, not only that, but its condition 

 furnishes a perfectly valid argument in favor of its younger 

 age. Such evidence as exists that it is the oldest, rather than 

 the youngest of the intrusives, has already been given in detail, 

 and, if this be the case, other evidence must be produced to 

 account for its apparent lesser degree of metamorphism. This 

 evidence is of threefold character and derived from the distri- 

 bution, the original texture and the composition of the rock. 

 The main anorthosite of the district occurs as a single great 

 intrusive mass, while the other intrusives are found in a num- 

 ber of smaller, disconnected masses, hence, for the reason just 

 outlined, the rude relationship between size of intrusion and 

 thoroughness of metamorphism, the anorthosite should in gen- 

 eral be less metamorphosed than the others. 



The anorthosite was originally a vastly more coarsely crystal- 

 line rock than were any of the other intrusives. The granula- 

 tion of these rocks begins at the margins of the separate crys- 

 tals and works inward by degrees, so that the amount of granu- 

 lation necessary to completely obliterate the original crystals 

 is, other things being equal, dependent on their size, since the 

 smaller they are the more margins there are where the process 

 can be initiated, and the greater the rapidity of destruction. 



The anorthosite has also a simpler mineral composition than 

 the other intrusive rocks, since it is mainly or wholly made up 

 of the one mineral, labradorite. Hence there is afforded com- 

 paratively little opportunity for the development of new min- 

 erals by corrosion, so that recrystallization has not gone on to 

 the extent that it has in the other rocks, and in so far as it 

 has occurred, can mostly only give rise to more labradorite. 



