410 Scientific Geology. 



its real composition : and I have even had a suspicion whether it may 

 not be the hypersthene rock of Macculloch. 



7. Porphyritic. There is considerable diversity in the composition 

 of the rocks included under this term. Their characters and situation 

 deserve a particular notice, since they are frequently useful for orna- 

 mental purposes. (1156 to 1164.) 



On Cape Ann a variety occurs, which resembles the black porphy- 

 ry of the ancients, and appears to be the trap porphyry of Werner, 

 and the melaphyre. of Al. Brongniart. I should describe it as having 

 a base of common greenstone, with large imbedded crystals of green- 

 ish foliated feldspar. Sometimes these crystals are more than an inch 

 in diameter. It occurs at Sandy Bay, near the village, in veins in 

 sienite. A similar rock is found in veins at Marblehead, according 

 to the Messrs. Danas; also in rolled masses in Dorchester, Brook- 

 line, and Roxbury. I have noticed the same rock in rolled pieces in 

 Easton, except that the feldspathic crystals are white. (Nos. 1156, 

 1157.) 



In Ipswich, west part, I found a rolled mass which appears to be a 

 greenstone with numerous foliated masses of a shining black color, 

 which I at first suspected to be feldspar : but I am satisfied that they 

 are Karinthin. (No. 1159.) 



A considerable part of the eastern or upper side of the greenstone 

 in the Connecticut valley, is very different in its composition from the 

 principal part of the ridges. The basis of the rock is wacke-like ; 

 and some of it is amygdaloidal, and some of it porphyritic. The fo- 

 liated masses of feldspar, however, are so small and so numerous, 

 that I doubt whether it might not with quite as much propriety be 

 reckoned as common trap. I doubt whether it contains any horn- 

 blende. Its general color is gray. (Nos. 1163, 1164.) 



Occasionally we meet among the greenstone of that part of the 

 State, with other varieties that are more or less porphyritic. No. 

 1161, from Deerfield has a compact homogenous base, nearly of the 

 color of brick, with a few small imbedded crystals of feldspar. It is 

 found in the same mass with common greenstone ; but seems to have 

 been exposed to a higher degree of heat. No. 1160, from Turner's 

 Falls, has a variegated base, whose, nature is not obvious, with crys- 

 tals of feldspar. 



8. Amygdaloidal. This structure, like the porphyritic, is found in 

 nearly all the varieties of greenstone that have been described. The 

 following are the most common of our amygdaloids. (Nos. 1166 

 to 1175.) 



