190 



Scientific Geology. 



between the trap and sandstones : but in such cases, I apprehend that 

 the fusion of the trap was only partial, as we know to be the case 

 with many lavas. 



Further : how could nodules of iron, in the midst of clay and sand 

 beds, have been exposed to a fusible heat, and yet the clays and sands 

 have remained unconsolidated ? 



3. Mamillary. The tubercles of this variety are rarely smaller 

 than a buck shot, or larger than an ounce ball. When broken, they 

 exhibit no appearance of a radiated structure ; but are massive, though 

 scales of mica and grains of sand are observable in every part of this 

 ore. (No. 120.) I cannot find that this variety of iron is described 

 in the books on mineralogy. I am informed by Thomas A. Green, 

 Esq. that it exists in much larger quantities in the cliffs, four miles 

 east of Gay Head, in the west part of Chilmark, a little east of Mo- 

 nimshi Bite, than at Gay Head, where I found it. 



4. Pisiform. The grains are rarely exactly spherical, and seldom 

 exceed the size of a small pea. Not unfrequently they are distinctly 

 reniform. Color, externally, blackish brown ; internally, yellowish 

 brown. Aggregated into irregular masses with clay and gravel. 

 This ore seems sometimes to be the mineralizer of alcyonia, shells, 

 &c. It is not abundant. 



5. Ochrey Brown Iron Ore. This occurs mixed sometimes with 

 every variety above described ; particularly with the nodular. But 

 it is never seen in large masses. 



It is well known that the preceding are valuable ores of iron for 

 smelting ; and at Gay Head, particularly, they may be obtained in 

 abundance. That spot is still" in possession of the descendants of the 

 Indians of Martha's Vineyard: and it is to be hoped that the Gov- 

 ernment of the State will take measures to prevent their being de- 

 frauded of this ore, which may prove of considerable value. 



Radiated Sulphuret of Iron. This is very frequent and beautiful 

 in the osseous conglomerate and blue clay of Gay Head. The nod- 

 ules are sometimes perfect spheres ; from one to three inches in di- 

 ameter ; but generally more or less irregular ; the surface often ex- 

 hibiting one face of numerous cubic crystals : but on breaking the 

 mass, the radiation is obvious. Such, however, is the tendency of 

 this ore to decomposition, that it is difficult to preserve specimens long 

 in a cabinet, unless excluded from the atmosphere. Their decompo- 

 sition produces of course sulphate of iron, and sulphate of alumina 

 and potash, or alum, which effloresce on the clay. 



