112 



quently, it is not uncommon, in traversing the dioritic areas, to 

 find limited beds of quartzite which still preserve their normal 

 strike and dip, although enclosed on every side by eruptive or 

 semi-eruptive diorite. Such masses, when the bedding is indis- * 

 tinct, sometimes have the appearance of huge veins traversing 

 the diorite. One of the largest of these quartzitic islands is 

 well exposed in the vicinity of Shaker Glen, on Mill Brook, in 

 Woburn. It is a beautifully and evenly stratified quartzite, or 

 something between quartzite and petrosilex. The compara- 

 tively large area of stratified rocks enclosed in the diorites in 

 the north-east corner of Weston is itself largely dioritic, con- 

 sisting of fine-grained, slaty rocks. The foregoing remarks 

 concerning the occurrence of stratified rocks in the Salem and 

 Marblehead and the Stoneham and Weston areas of diorite are 

 true in a general way of all the dioritic areas. 



The conclusion is now certainly safe that the eruptive diorites 

 and the stratified group are unequally metamorphosed portions 

 of one great series of basic rocks. The metamorphism reaches 

 its extreme in the southern part of Norfolk County and in the ' 

 Salem and Marblehead area ; and it is the general law that, in 

 each of the great belts of basic rocks, the metamorphism 

 increases southward or south-easterly, the exotic lying mainly 

 to the south of the stratified portions. This is most noticeable 

 in the northern part of Essex County, and in the region between 

 Essex and Wayland. Now, since the normal dip of the entire 

 Huronian formation, and especially of this portion of it, is 

 toward the north and north-west, it follows that in the main the 

 dioritic or eruptive group underlies the stratified group; i.e., 

 the metamorphic action has affected the lower terranes of this 

 great basic series more than the upper. 



LIMESTONE. 



The Huronian limestone of Eastern Massachusetts also 

 belongs to the stratified group. It is the least abundant of all 

 our Huronian rocks. Prof. Hitchcock showed that it is always 



