STRATIGRAPHICAL GEOLOGY. 



Sections. 



I present herewith three sections passing across the most important 

 parts of the field of the map. Taken in connection with the map, they 

 will afford a correct notion of the geology of this district. 



Geological Section from PlynioiitJi^ jMass., to WJiitcJiaU., N. 1'. 



This section is i8o miles long, and exhibits the older rocks of eastern Massachusetts, 

 the Atlantic group in New Hampshire and Vermont, the so-called Taconic rocks in 

 western Vermont and eastern New York, the Adirondack or Laurentian series, and 

 several less important groups. 



Underneath the drift of Plymouth and Kingston there lies a broad band of granite, 

 the north-eastern extremity of the range continuous from Little Compton, R. I. The 

 descriptions given of it are not sufficient to enable me to compare it directly with any 

 of the New Hampshire groups, though there are suggestions in favor of a reference of 

 certain portions to the Bethlehem and Labrador groups. Nor is it known how this 

 band stands related to the sienite, which appears next in Pembroke. The Carbon- 

 iferous system which succeeds is the northern extremity of the New England anthracite 

 coal-field. The strata are largely slates and sandstones, with a moderate northerly dip, 

 and rest upon a floor of sienite. No beds of anthracite are known in this neighbor- 

 hood ; and the few exposures may belong to the lower part of the group. It is the 

 newest formation exhibited upon the section. 



The sienite is the fundamental rock of south-eastern Massachusetts. It comes to 

 the surface four times in our route of travel. Being an eruptive rock, it is evidently 

 newer than some of the neighboring gneisses, either the New Bedford range (which 

 may possibly touch the end of our section) or that which appears between Sudbury and 

 Harvard. Not all this rock, however, is to be regarded as eruptive. It merges into the 

 hornblendic gneiss of Wayland and Sudbury, and is often traversed by divisional 

 planes very suggestive of strata. The group also contains beds of limestone and ser- 

 pentine, and there are conglomerate portions. As the limestones of this series and the 

 gneiss of Bolton carry the organism Eozoon, the whole series is evidently allied to the 

 Laurentian. The geographical situation of the sienite and eozoonal gneisses naturally 

 suggests a Laurentian age, for they may belong to the Acadian ranges of these older 

 rocks, and they certainly lie to the cast of all the debatable metamorphic formations. 

 The similarity of the sienites to rocks among the White Mountains, described else- 

 where as cutting the Labrador sediments, as if at the termination of their period of 

 formation, affords reasons for supposing them to be of the same age. Thus we are con- 

 fronted with two theories of the age of the sienite. Were it possible to decide the 

 question of age without further field-work, it might be said that the apparently stratified 

 IDortions may be of Laurentian age, while the other masses may have been erupted in 

 the Labrador period coeval with Red hill and Tripyramid. The probability of the 



