CAMBRIAN ROCKS 



297 



conglomerate passes upward into a gray compact crystalline quartzite, 

 which in turn grades into a silicious slate or into a sericitic quartz- 

 schist, or is altogether replaced by the quartz-schist. 



The sericitic quartz-schist is typically exposed in quarries one-half 

 mile northeast of Somerton station and in the Edge Hill quarries. It is 

 a thin bedded formation here. Bedding and cleavage coincide and dip 

 steeply southeast. The quartz schist is of a light buff to white color and 

 always contains feldspar. The feldspar is for the most part orthoclase, 

 more rarely microcline, and is usually more or less kaolinized. Tour- 

 maline, apatite, zircon, magnetite, and staurolite are accessory constitu- 

 ents. Stretched tourmalines are very characteristic of the formation. 

 The schist readily cleaves into slabs and, by means of fissility developed 

 in shear planes, into flattened rhombohedrons. Locally the quartzite 

 may be quite geodiferous. The geodes are lined with quartz crystals. 

 Quartzite of this character occurs at a locality known as " Diamond 

 rock," on the southeast flank of the north Chester Valley hills. A 

 chemical analysis shows the rock to be highly silicious, with sufficient 

 alumina and potassa for sericite and lime-soda-feldspar. Analyses III 

 and IV are of very sericitic facies of the quartz-schist. 



Cliemical analyses of Chickies quartzite 



I. II.* III. IV. 



Si0 2 87.87 84.59 58.97 5G.35 



A1 2 3 6.61 .... 22.61 22.28 



Fe 2 3 2 39 .... 5.67 3.21 



MgO Trace 0.25 1.40 



CaO 0.24 .... 0.08 0.19 



Na 2 0.19 0.19 0.32 0.38 



K 2 1.73 2.79 7.34 12.63 



Ignition 1.20 1.66 3.73 2.89 



Ti0 2 0.38 .... 1.11 0.82 



P 2 5 0.06 .... 0.07 0.16 



MnO 0. 13 Faint trace Trace. 



Li 2 Strong reaction. 



100.80 .... 100.15 100.31 



I. " Itacolumite," 1£ miles southeast of Vanartsdalen's, near Neshaminy creek. 



II. " Itacolumite," east bank Neshaminy creek. 



III. "Itacolumite" from quarry northeast of Somerton. 



IV. One-half mile south of Willow Grove station, Montgomery county, Penn- 

 sylvania. Analyses made by F. A. Genth, Jr., volume C 6, Second Geological 

 Survey of Pennsylvania, pp. 107, 117, 121. 



* Partial analysis. 



XL— Bum.. dmh. Soc. Am.. Vol,. 10, 1004 



