412 



Scientific Geology. 



empty, the rock can hardly be distinguished from some recent lavas. 

 (Nos. 1170 to 1174.) 



All these varieties with a wacke-like base, exhale a strong argilla- 

 ceous odour when breathed upon. 



The greenstone in the eastern part of the State is rarely amygda- 

 loidal. 



9. Concreted* The argillaceous substance above described, as form- 

 ing the base of certain amygdaloids, sometimes contains numerous 

 distinct concretions of the same substance, apparently more indurated. 

 They are generally spheroidal ; and the concentric crusts not more 

 than a line in thickness. Sometimes I have observed the central nu- 

 cleus to consist of a rounded mass of amygdaloid enveloped by coats 

 of indurated clay or wacke. The diameter of these concretions is 

 sometimes six or eight inches: but usually not more than two or three. 

 They sometimes occur imbedded in the next variety to be described. 

 Their most abundant localities, which I have noticed in Massachu- 

 setts, is in Deerfield, east of the village, and on Mount Holyoke, near 

 the road from Amherst to Granby. (Nos. 1176, 1177.) 



10. Tufaceous. This embraces all those rocks that are composed 

 of fragments of any of the varieties of greenstone that have been de- 

 scribed ; whether these fragments are angular or rounded ; united by 

 " trap sand," or the same materials in a comminuted state. Sometimes, 

 however, the rock consists of angular fragments of greenstone, cement- 

 ed by calcareous spar. In this case it is obvious that the spar was intro- 

 duced by watery infiltration, after the fragments had been piled togeth- 

 er. In other cases, it is equally obvious that the fragments have been 

 melted together : for we distinguish the different materials of which 

 they consist, only by the different colors ; it being no easier to sepa- 

 rate the rock along the line where the fragments unite, than in any 

 other direction : and I do not suppose it possible to unite fragments 

 so firmly except by fusion. (Nos. 1 178 to 1186.) 



Topography of the Greenstone. 



The parts of the State in which greenstone occurs in sufficient 

 quantity to be noticed on a map, are only two. In the eastern and 

 northeastern part of the State, it will be seen that there are extensive 

 ranges. As we pass beyond the gray wacke and argillaceous slate 

 that encircle Boston on the north, west, and south, we usually find 

 greenstone to be the predominant rock. Even on the south, in Mil- 

 ton, &c. where porphyry is represented on the map as succeeding to 



