406 GEOLOGY OF DELAWARE COUNTY. 



and recrystallize the feldspar in the adjacent gneiss without destroying its lami- 

 nated structure. The effect of this re-crjstallization on the durability of the 

 feldspar has been very remarkable. It has now become the least destructible 

 of the three ingredients of the rock ; the crystals of feldspar appearing as a 

 gravel in the soil resulting from the decomposition of this porphyritic gneiss. 

 The fertility of this soil is unsurpassed — as it is constantly supplied with potash 

 from the very slow but gradual decomposition of the feldspathic gravel. As 

 another evidence of the increased durability of the feldspar in this metamorphic 

 gneiss, I may mention that in the exposed boulders of this rock, the feldspar 

 crystals are found standing in relief. 



The trap composing this dyke varies greatly in its appearance, and doubtless 

 somewhat in its composition. It generally has the aspect of coarse basalt. Its 

 decomposition has been more rapid than that of the adjacent rocks, but still 

 there are a sufficient number of spherical trap boulders on the surface to mark 

 the course of the dyke and itg tranches, that yield but slowly to the weather, 

 and scarcely at all to the sledge. The branches from the main trunk generally 

 pass southerly, effecting a metamorphism of a wide belt of gneiss, or that meta- 

 morphism is owing to other causes not apparent. Some of this metamorphic 

 rock exhibits alternate lamina of light-colored crystalline feldspar, with dark fine- 

 grained mica and cjuartz, affording a valuable building stone. In many other 

 places the matamorphism has almost destroyed the rock for economical purposes, 

 while in a few spots the gneiss has not been in the least affected. Large granitic 

 veins presenting their usual coarse appearance, have had a share in changing 

 the character of the rock a little further south and east. These veins are abun- 

 dant about Kellyville. Below a line commencing at Cobb's Creek, near the 

 Delaware County turnpike, and crossing Darby Creek some distance above the 

 town of Darby, the gneiss becomes more micaceous, assuming the character of 

 mica schist in some places, as may be seen on Church lane, near Cobb's Creek. 

 On Mount Zion Hill, near Darby, this micaceous rock takes in as an ingredient a 

 small portion of kyanite. These more micaceous strata, however, alternate with 

 those that are less so, and in which the mica is dark-colored, or its place is sup- 

 plied by hornblende. At Kellyville the gneiss rock in some of its strata is highly 

 ferruginous, and at Darby still more so. 



Below the " water shade" the underlying rocks are so much covered with dilu- 

 vium, and the earthy matter resulting from their own decomposition, that it is 

 exceedingly difHcult to trace the direction of their strata with any great degree 

 of certainty. Isorth of Darbj' Creek Ferry we find the strata micaceous, and in 

 one belt of considerable dimensions, the place of the mica is nearly altogether 

 supplied by dark-colored kyanite. 



In the immediate vicinity of Darby Creek Ferry, and several other places in 

 the same neighborhood, the exposures of large granitic veins, of the very coarsest 

 materials, arc numerous. One of these is on the island of Tinicum, half a mile 

 above the ferry. In all these veins feldspar is the most abundant material. 



Below the post road, and near Little Crum Creek, the gneiss strata become 

 more solid, the proportion of quartz increased, and the mica, which is dark- 

 colored, less abundant. Plere quarries have been opened, from which large 

 quantities of stone were taken to the Delaware Breakwater. These quarries are 

 not in the range of strata in which nearly all the well-known gneiss quarries of 

 Delaware county are located. 



Near Lieperville, but still southeast of the regular gneiss quarry range, are found 

 strata of no great dimensions, but abounding in silicious particles, almost to the 

 exclusion of the other ingredients of gneiss. These particles are sharp and 

 gritty, and some years ago large quantities of the partially decomposed rock 

 were manufactured into scythe stones, and were well known under the name of 

 " Crum Creek ytones." 



The large trap dyke that has been mentioned, disappears in Springfield, and 

 the southwestern part of that township, with the northwestern part of Ridley, 

 the southern part of Nether Providence and the eastern part of Chester town- 

 ships being, in a great measure, free from large intrusive veins of granite, the 

 gneiss is there found uudisturljed and unaltered, and generally of an excellent 



