410 GEOLOGY OF DELAWARE COUNTY. 



Chester county. This blue quartz is best seen in the decomposed rock where its, 

 small grains form an angular gravel. Specimens weighing several pounds are 

 sometimes met with. 



North of the gneiss of Radnor the rock is chiefly a talcose slate, containing 

 much (juarlz and i? in a highly metamorphic condition. This forms what is 

 known as the '-Gulf Hills." At 'the foot of these hills, near the Gulf Creek, a 

 small deposit of impure limestone exists within the limits of our county. 



A notice of the numerous exposures of serpentine within the county, only the 

 larger of which will be particularly described, will close our account of the un- 

 derlying rocks The map will show, with a good degree of accuracy, the loca- 

 tion of each exposure. 



Mr. Rogers has overlooked every out-crop of serpentine within the limits of 

 our County, except one. which is about half a mile from Morgan's Corner, in 

 Radnor township, and which is very inconsiderable in extent. This exposure is 

 300 feet wide and contains, according to Mr. Rogers, " true injected or igneous ser- 

 pentine, and serpenlinous steatitic ialc-slale." It is not believed that this serpentine 

 has any connection with any other exposure of the same rock within the County. 

 There is. however, a small exposure in Lower Merion township, just beyond the 

 Radnor township line, that appears to be continuous with the extensive develop- 

 ment on the borders of the townships of Radnor, Newtown and Marple, and 

 serves to connect this serpentine with the serpentine and steatite of Mill creek, 

 and that of the Schuylkill at the soapstone quarry. The development mentioned 

 is located on the line of separation between the micaceous district and the 

 northern gneissic, and for a considerable distance forms the boundary between 

 them. This development of serpentine embraces both the stratified and uustra- 

 tified rock, and is associated with steatitic rocks, and also with true talcose slate 

 in small quantities. 



As is the case with every extensive development of serpentine in the county, 

 this one is accompanied with a trap dyke. A little northwest of the residence 

 of Henry Hippie, in Marple, this dyke at one point is beautifully exposed, and in 

 conjunction with another unstratified rock, highly crystalline in its character, 

 forms a hill of no mean proportions. The fresh fracture of this rock is of a 

 dark green color, and its crystals are so interlaced as to give it such a degree of 

 toughness as to render its fracture very difficult. Its appearance is intermediate 

 between pyroxine an(?tremolite. Prof. Booth, of the U. S. Mint, judging from a 

 hand specimen, has rather doubtfully decided to call this rock diallar/e, a variety 

 of aiiyite. while J. C. Trautwine, Esq., who visited the locality, unhesitatingly 

 named it anthophi/llile, a variety of hornblende. The distinction between augite 

 and hornblende is very slight. Whichever of the names may be the more ap- 

 propriate, the vast extent of the rock, will be a surprise to geologists and mine- 

 ralogists who may visit the locality. Diallage is not abundant in this country, 

 while anthophyllite has been regarded as rather a scarce mineral. This locality 

 will furnish enough of the material to build a city. The exposure of the rock at 

 the top of the hill is now nearly excluded from view by the dense growth of 

 young timber. It is shown, however, sufficiently well in the annexed litho- 

 graphed drawing. The great number of boulders of trap and of this associated 

 rock that lie scattered over the side of the hill, attest the force with which the, 

 matter of these rocks was upheaved. 



This new unstratified rock also constitutes a part of the rocky exposure in 

 Edgmont township, well known as C'aslle Rock, but the connection between it 

 and the trap cannot be so well seen at this place as at the exposure first men- 

 tioned. 



The new rock as it appears in the dyke near Hippie's, and in the scattered 

 boulders round about, is strictly unstratified, but isolated rocks very similar iu 

 ajjpearance are found adjacent to this exposure of serpentine in which a lami- 

 nated structure is very apparent. I have not met with these laminated rocks in 

 place. 



Near the Yellow Springs road, bordering this serpentine, there may be seen 

 a tliin stratum of gneiss, very fine in its texture, and of an unusually white 

 color. This rock, upon close inspection, will be found to have taken into its 

 composition a portion of light-colored talc. 



