GEOLOGY OF DELAWARE COUNTY. 407 



quality. It is within the limits that I have mentioned that all the noted gneiss 

 quarries of Delaware county are found. Isolated patches of a similar rock that 

 have escaped tlie influence of secondary metamorphic agencies, arc met wilh in 

 other i)arts of the County, hut they are of limited e.\tent, and being inconvenient 

 to tidewater, quarries opened in them can only serve for local purposes. 



The quarries on Crum and Ridley Creeks are on or near tidewater, affording 

 every facility for tran.«porting their products to market, and for wliich the city of 

 Pliiladelphia, almost from the time it was founded to the present time, has 

 afforded one both convenient and ready. 



Though tlie structure of these rocks, and the minerals entering into their com- 

 position, bring them up to the standard of true gneiss in the strictest sense of the 

 term, still the usual varieties of that rock are found in the dill'erent quarries. 

 (Jn the one hand it approaciies granite so nearly as not readily to be distin- 

 guished from that rock, and is known in commerce under the name of granite, 

 while on the other hand it similates mica schist, retaining, however, its qualities 

 of hardness and durability. The former is cut and employed for every architec- 

 tural purpose for which granite is adapted, while the latter, besides being exten- 

 sively quarried as a building stone, supplies nearly all the curb-stone used in the 

 city of Philadelphia, and some for other places. For this latter purpose no 

 stone could be better adapted ; its hardness and toughness fitting it for the 

 severe usage to which it is subjected on the sides of the streets, — while numer- 

 ous seams, nearly at right angles with its cleavage, and at suitable distances 

 from each other, enal)les the skilful workman to take out blocks of nearly the 

 exact size wanted, and of lengths varying from five to forty feet. 



Immediately west of Ridley Creek, and for some distance above the post road, 

 and probably for some distance below it also, the character of the rock is essen- 

 tially different from that in its immediate vicinity. As exhibited in Spencer 

 Mcllvain's quarry above the road, it is substantially composed of hornblende and 

 quartz intimately blended, and has a specific gravity of 3.13. 



"We have now considered the underlying rocks of the southeastern section of 

 the county, below the develo[)meut of trap first mentioned, and an imaginary 

 continuation of it to the neighborhood of Chester. On the upper side of this 

 trap much less change has been effected in the character of the adjacent strata : 

 still a change is well marked in many places. In proceeding northward from 

 the line already designated, the strata gradually become micaceous, until they 

 pass into well marked mica schist in the eastern part of the County. Before 

 reaching this point, strata of hornblende schist and silicious schist, alternate 

 with micaceous gneiss, or with true mica slate. In the southeastern part of 

 Marple township, a few narrow strata occur, composed chiefly of a sharp sili- 

 cious sand, with minute particles of mica interspersed. The lithological charac- 

 ter of this rock, as well as its immediate associations, would entitle it to be 

 classed as a mica slate. It is from these subordinate strata that the well known 

 " Darby creek scythe stones " are manufactured.* 



The map will exhibit approximately where gneiss predominates on the one 

 hand, or mica slate on the other; but it may be repeated that the colors of the 

 map only indicate the predominance of a rock, and not lis presence, to the exclu- 

 sion of all others. 



The next belt of strata to be noticed is that in which all other rocks may be 

 considered subordinate to mica slate. This belt occupies quite a considerable 

 district in our County, bnt has attracted little attention, from the fact that a 

 large part of it is hidden from view by the remains of an ancient deposit of clay, 

 sand, and gravel that rests unconformably on the micaceous strata which compose 

 it. This deposit has mostly disappeared along the streams; which circumstance 

 allows us to form a good judgment of the general character of the underlying 

 rock, though its particular features in many places must remain a matter of 

 conjecture. 



For the boundaries assigned to these micaceous strata, which are in a measure 



• The stone, after having been removed from the quarry, is split into blocks of nearly the right 

 size These are dressed into shape with a kind of hatchet, after which the stones are finished by 

 grinding iheoi on a large stone running horiz ntally, with pewter sand and water. 



