310 On the Constituents of PHmary Rocks, 



and stretching to the south and west, fronts the Delaware river, 

 as far as Wilmington, in the State of Delaware ; whence it can 

 be traced inland, in the neighbourhood of Baltimore, and much 

 farther into the southern states. The varieties of these horn- 

 blende rocks are very great ; and as it is of these the Delaware 

 Breakwater is now constructing, w^e have thought it due to the 

 communication which Major Bender has favoured us with, to ac- 

 company his table of specific gravities, with some remarks on the 

 mineral nature of these rocks. Having personally visited most 

 of the localities mentioned in this table, we have had occasion to 

 observe how generally the erroneous designation of trap, is given 

 to some of the varieties of hornblende rocks, and as some of our 

 correspondents have also requested information from us on this 

 subject, we have thought to render a service to our readers by 

 entering into such details of the primary rocks, as may enable 

 them to judge with success for themselves, of the proper names 

 to give those varieties which fall under their observation. We 

 have spoken of felspar as forming the principal mineral in granite, 

 with quartz and mica ; when it is compounded with the mineral 

 called hornblende, it constitutes that class of rocks of which we 

 have spoken as extending from Philadelphia to Wilmington. 

 Hornblende, called by the French, amphibole, is heavier than 

 quartz or felspar, and when scratched, gives a light green streak. 

 It contains a great proportion of magnesia, which felspar has 

 not; and when the quantity of magnesia is increased, it passes 

 into serpentine. The Germans call these combinations of felspar 

 and hornblende, griinstein, or greenstone, especially when they 

 have a granitic structure. When hornblende forms the princi- 

 pal part of such rocks, they take a greenish black colour. When 

 it is combined in lamellar grains with felspar, it is called sienite. 

 In some instances, as at Quarryville, on the Delaware, near 

 Wilmington, the felspar is in beautiful resplendent lamellar crys- 

 tals, of an oval form, and of a lightish red colour. This in the 

 common language of mineralogy may be called a porphyrytic 

 greenstone. 



We have remarked, that the erroneous designation of trap has 

 been given to these hornblende rocks ; and this, no doubt, has 

 grown out of there being an intimate combination, in some in- 

 stances, of hornblende and felspar. This is also the case with 

 the rocks which have received the generic name of trap, from . 



