Varieties of Greenstone. 



409 



The two varieties that have now been described, comprehend the 

 greater part of the grunstein of Werner, the diorite of Hauy, and 

 the diabase of Al. Brongniart. These writers, however, mention 

 only compact feldspar as an ingredient : but Dr. Macculloch very 

 properly adds common feldspar. 



3. Compact. In this variety, which is almost entirely homogene- 

 ous and finely granular in its texture, the different ingredients can- 

 not be distinguished. In some cases it is probably only indurated 

 clay, or wacke, with some dark coloring matter : in other cases, it 

 may be hornblende and feldspar, completely melted together. The 

 aspect of the rock approaches closely to some varieties of basalt ; but 

 it is doubtful whether we have any trap rock in this part of America, 

 which was produced at the same epoch, or is composed of precisely 

 the same mixture, as the European basalt. The variety under con- 

 sideration occurs generally in the form of veins; as at Nahant, &c. 

 (Nos. 1140 to 1143.) 



4. Chiefly Greenish Compact Feldspar ? This is a beautiful rock ; 

 but its characters are very obscure. Perhaps it ought to be described 

 under porphyry : but its great resemblance to the traps, has led me 

 to place it here. It oecurs in Essex county along with sienite, com- 

 mon greenstone, &c. (Nos. 1144 to 1147.) 



5. Indurated Clay. This variety is of limited extent; occurring 

 only at the junction of greenstone and shale ; as at Titan's Pier. In 

 aspect it approaches to hornstone ; being of a light gray color. In 

 the same mass with this rock, we usually find angular pieces of com- 

 pact trap : so that in fact, it might have been described under trap 

 tufa. (Nos. 1148 to 1150.) 



6. Hornblende, Augite ? and Feldspar. The hornblende in this 

 compound is in crystalline fragments; and the mineral which I sus- 

 pect to be augite, is of a greenish aspect, but scarcely crystalline. 

 The feldspar is sometimes foliated and in small quantity. It occurs 

 only at Nahant, a little distance northwest of the Hotel: and the most 

 remarkable circumstance in relation to it, is its apparent regular strat- 

 ification. This is the only instance that I know of in Massachusetts 

 where a trap rock exhibits those parallel divisions. I do not, howev- 

 er, regard them as real strata, for reasons that will be hereafter 

 mentioned. (No. 1152 to 1155.) 



I should not have noticed the above as a distinct variety, had not 

 its peculiar aspect excited the suspicion that it might be a dolerite of 

 the geologists of continental Europe. I do not feel satisfied what is 

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