DISTRIBUTION OF PYROPLASTIC ROCKS. 
109 
The feldspar is rather concretionary than crystallized. The 
white spots are frequently quartz. 
§ 81. Trap dykes are those black stony masses which are 
interposed between the walls of a fissure. For a limited dis¬ 
tance they pursue a straight course. The fissure is perfectly 
Fig- 23. defined, and the filling always perfect. 
Where more than one dyke intersects 
a rock, they may be parallel, as re¬ 
presented in fig. 23, or they may in¬ 
tersect each other. In the first case 
the dykes are probably of the same 
age, but of a later date than the masses 
which they intersect. The rock tra¬ 
versed by these dykes is hypersthene, 
a A mass of Pyrocrystalline . „ .. , ,. 
Limestone, b of Granite, c a 9il th.6 SUDOldlllcLtc IH9SS6S 9.16 
vein of Magnetic Iron Ore, ; Kneous products, 
a a four parallel Trap Dykes, ° r 
which may be traced a hun¬ 
dred yards. 
DISTRIBUTION OF THE SUBMARINE PYROPLASTIC ROCKS 
IN THE UNITED STATES. 
§ 82. In the eastern section of this country three belts of 
greenstone are well known. They belong to the eastern or 
Atlantic slope of the Appalachians and the Green or Hoosick 
mountains, and are coextensive with them. Two of these belts 
are parallel, and were synchronously erupted. The eastern belt 
begins in Rhode Island, and extends entirely across the eastern 
part of Massachusetts into New Hampshire. The belt is promi¬ 
nently exhibited in Weston, Watham, Lexington, Woburn, 
Wrentham, and onwards to Ipswich in New Hampshire. The 
direction of this belt, upon the whole, bears to the eastward. 
It is not, however, in a distinct belt or ledge, but a broad area 
in which these eruptive rocks are common. It is intimately 
associated with sienite, another eruptive rock which accompa¬ 
nies it through its whole route. The two masses form, as it 
were, a large patch of rocks, encircling in part Massachusetts 
