X 0/2 the Constituents of Primary Rocks, 311 



their dividing into prismatic forms, and forming steps or stairs. 

 (Trappa, in the Swedish tongue, means a stair.) Cabinet speci- 

 mens of these respective rocks, sometimes resemble each other 

 so closely, that they would puzzle a good practical geologist to 

 decide whether they did not belong to the same class of rocks. 

 There is also another mineral, augite, which combines with fel- 

 spar in the same manner that hornblende does, and which is dif- 

 ficult to distinguish from it. The dark black basalts, which 

 geologists are now agreed, have the same origin as the true trap, 

 are composed of felspar and augite, finely combined, with some- 

 times grains of the mineral called olivine, and black oxide of iron. 

 However these greenstones may resemble in their constituent 

 particles, the traps — now universally admitted to have had an 

 origin of the same nature with lava, of modern times — an expe- 

 rienced geologist can at once decide when he observes them 

 aperto campo. Nothing can be more dissimilar with the massive 

 hornblende rocks, fronting the Delaware river, — and undoubtedly 

 associated with the primary rocks, — than the true trap on the 

 Hudson river, at the Palisades, that at Hartford and New Haven, 

 in Connecticut, and that at the Passaic falls. New Jersey, all 

 of which overlie secondary rocks. To call the hornblende rocks 

 then, of which we have been speaking, trap, is to confound very 

 important geological distinctions. The various combinations of 

 felspar and hornblende, and felspar and augite, have produced 

 the rocks called greenstone, sienite, trap, and basalt ; together 

 with all the varieties which a change in the proportion of con- 

 stituents occasions, such as are clinkstone, pitchstone, amygdaloid, 

 and other porphyries. 



To these rocks formed of hornblende and felspar, the French 

 have given the wame oi diabase ; and to those basaltic compounds, 

 into which augite enters, they have given the name of dolerite. 

 We know of no name more appropriate to the rocks we have 

 been considering than hornblende rocks, because hornblende is 

 chiefly found combined with felspar, when associated with the 

 primary rocks ; whilst augite is more peculiar to rocks of ac- 

 knowledged volcanic origin, although hornblende is also found in 

 them. The term diabase, is applicable to any rock having a 

 double base, and we, therefore, prefer a name that expresses at 

 once the mineral to which the rock owes its distinctive character. 

 ^Ve trust that this subject will receive proper attention from 



