1890.] NATURAL SCIENCES OF PHILADELPHIA. 103 



First. Liiurentian. 



Second. A porpliyritic gneiss resembling Rogers' altered primal 

 as found northwest of the Laurentian. 



Third. A micaceous schist in very small quantity. 



Fourth. Hornblende fragments. 



Fifth. Serpentine. 



The serpentine extends along the road several hundred feet. 

 North of it is Laurentian, but no contacts are visible. About a 

 quarter of a mile west of it appears the same rock spoken of as resem- 

 bling Rogers' altered primal, striking, like the former, N. 65° to 70° 

 E. but the dip not certain. Beyond this west to West Chester and 

 north for several miles the Avhole region is Laurentian. 



It will be seen that all these outcrops of serpentine are close to 

 the margin of the Laurentian, two of them at least within its mar- 

 gin, though there is no sufficient evidence to prove whether they are 

 of that age or more recent. 



INIr. Hall, as before stated, thinks it safe to assume that they are 

 among the most recent deposits, but the section of the Radnor belt 

 shows going northward from the Lau rentian axis several rocks between 

 the serpentine and the Potsdam sandstone, seeming to indicate that 

 here at least the serpentine is the oldest except the Laurentian, and 

 that the steatite is more recent than the serpentine but below the 

 Potsdam, unless in a synclinal. 



On the southeast side of the Laurentian the existence of the Pots- 

 dam is not certain where the serpentine occurs, but as above stated 

 it does occur at Waverly Heights in Montgomery county, between 

 the Laurentian on thenorthwestand the schists on the southeast and 

 there are in Radnor and Lower Merion certain rocks in the same rela- 

 tive position Avhich closely resemble Potsdam. If this is Potsdam 

 then the serpentine is again between the Laurentian and the Pots- 

 dam, but the very positionof the serpentine, so very close to the Lau- 

 rentian, would indicate that it w'as an older rock than the mica schists 

 to the southward. 



We come now to the consideration of the outcrops which are not 

 near the Laurentian. Of these perhaps the most important are the 

 steatite belts N. W. & S. E. of the Radnor and LaFayette serpen- 

 tines. 



THE NORTHERLY RADNOR BELT. 



Outcrops of this belt lie nearly parallel to the great Radnor belt 

 but extend a mile further northeast and all lie within a lineal dis- 



