200 



On the Trap Tuff of the Connecticut Valley. 



appear 



6. 



granite, quartz rock, clay slate, &c. These rocks 



or less crystalline in the structure of their cement, and contain, 



especially, foliated nodules of calcareous spar. 



3. Volcanic Breccia. — This is composed of angular fragments 

 of trap, of a highly ferruginous character, with a small proportion 

 of cement of the same character. This rock abounds in the large 

 ridges of intrusive trap in the vicinity. 



4. Amygdaloid, with a somewhat compact base, and nodules 

 of calcareous spar. The base has the aspect of indurated vol- 

 canic mud. 



5. Volcanic Slags. — These differ but little from the amygda- 

 loid, except in being more vesicular, and the cavities are empty. 

 They cannot be distinguished from recent vesicular lava, except 

 in not being so fresh. 



Concretionary Nodules. — These are made up of concentric 

 coats and are a dirty color and highly ferruginous. They are 

 rarely more than three or four inches in diameter. 



7. Common Greenstone. — This differs not at all from the 

 greenstone that constitutes the great mass of the trap of this val- 

 ley, except perhaps in being usually more compact. 



8. Porphyritic T> rip.— This takes distinct crystals of feldspar 

 into its composition, and has a gray base more argillaceous than 

 greenstone. The same rock occurs in connection with the prin- 

 cipal ranges of trap. 



Relative Position and Stratification of these Deposits. 



The usual dip of the strata of sandstone in the valley of Con- 

 necticut, is easterly, averaging about 15° or 20° ; and the large 

 ranges of trap appear to have been protruded to' the surface be- 

 tween the strata. Consequently they lie upon the sandstone as 

 seen on the west side of the range ; while on the east side, the 

 sandstone lies in an inclined position upon the trap, frequently 

 with a higher inclination than the sandstone beneath. The east 

 side of the trap, therefore, I call the upper side, and the west 

 side, the under side. Now it is upon the upper side only that 

 the trap tuff occurs. After receding a greater or less distance 

 from the trap range, we find the tuff interstratified with the 

 sandstone, as shown on the sections, figs. 1 and 2 ; and the tuff 

 is also, for the most nart. distinrt.lv stratify ™nfnrmablv to the 



Sandstone 



Trap. 



Fig. 1. 



Sandstone. Trap tufa, 



Sandstone. 



Section across Mount Tom from west to east. 



