Frazer.] •'" [Dec. 20, 



Contribution to the Lithology of Pennsylvania. 



On the Physical and Chemical Characteristics of a Trap occurring at 

 Williamson's Point. By Persifor Frazer, Jr. (1 colored plate.) 



(Read before the American Philosophical Society, Dec. 20th, 1878.) 



A thin vein of trap intersects the cliloritic rocks at Williamson's Point, 

 on the Susquehanna River in Lancaster Co. Pennsylvania, and near the 

 Maryland line. 



This trap dj'ke which cuts through the hard quartzose and chloritic rock 

 at Williamson's Point is peculiar in its isolation from known rocks of ig- 

 neous origin ; in the manner in which it is foliated transversely to its con- 

 tact planes ; and in its disappearing on its under side in a feather edge. Its 

 upper continuation is now obscure from the denudation of the rocks which 

 it intersects, but as far as it, can be followed it widens in an upward direc- 

 tion, and the uneven facade of rock against which it appears gives it the 

 semblance of being dislocated in places, but this is a deception of the judg- 

 ment. 



The rocks are here twisted in a most extraordinary manner, and this 

 twisting is more remarkable just south of the position of the dyke. A very 

 fine specimen of a portion of this vein, with both walls distinct and at- 

 tached on one side to the rock which it intersected, is No. 1760 in the col- 

 lection of the Geological Survey. 



An examination of this specimen will reveal the fact that the fissure has 

 not been exactly along planes of lamination, but truncates the tops of sev- 

 eral small waves into which the strata have been forced. 



A specimen of this trap was obtained and reduced to a thin section, of 

 which a representation as seen under a power of 400 diameters and in polar- 

 ized light has been very faithfully made by Mr. Faber. 



It was not found expedient in this drawing to imitate exactly all the de- 

 tails in any one field of view, but the more characteristic exponents of the 

 minute crystals were brought together from all parts of the slide and sub- 

 stituted for those less perfectly formed; cine regard being had always to the 

 proportions in which the several constituents of the mass manifested them- 

 selves. 



In the centre of the field is a large double, or multiple, columnar crys- 

 tal of labradorite to the bottom, and to the middle of which other smaller 

 crystals are attached — whether accidentally in contact or an oil' shoot in 

 the former case is not certain. 



In the upper left hand portion of the field a curious instance of the split- 

 ting of labradorite may be observed. It was at first thought, that the ap- 

 parent divergent curvature of the two branches of this crystal might be an 

 optical delusion, and that in reality two independent individuals were thus 

 accidentally in contact atone extremity. Under higher powers than that 

 here given, however, it proved to lie an actual ramification of the mass 

 from one common slock like the growth of twigs from the same branch. 



The otlur labradorite crystals will be easily distinguished by the eye, 



