GEOLOGY OF DELAWARE COUNTY. 40!) 



Chester, and extend into Delaware State. On the middle branch of Naaman's 

 Creek they occur in the greatest profusion. 



In the vicinity of Village Green, ordinary micaceous strata are observable. 

 Further north, in Aston, above the " water shade," the gneiss strata are highly 

 metaniorphic, and appear in great confusion. The plutonic agency that caused 

 this metamorphism (doubtless protruded trap), has elevated hills of some height, 

 and has given a general unevenness to the surface of the country. Still further 

 north in Aston and Concord the gneiss is less altered, and is not characterized 

 by anything remarkable. 



Purposely passing over without consideration the several exposures of serpen- 

 tine in our county, with the view of considering them and their accompanying 

 rocks together, I will now proceed to notice the gneissic strata lying west and 

 northwest of the more micaceous belt that has just been noticed. These strata 

 are observed to be in a highly metamorphic condition in many places, and the 

 cause of this metamorphism is readily found in the numerous exposures of trap 

 rock that meet the eye in many places. This gneissic belt occupies nearly the 

 whole of the townships of Radnor, Newtown, Edgmont, Thornbury, and parts 

 of Upper Providence and Middletown. A part of Radnor township chiefly north 

 of the Gulf Creek, Mr. Rogers includes in the lowest member of his ancient 

 Palaeozoic strata. It will be noticed hereafter. Beyond Radnor, westward, the 

 northern boundary of the gneissic district, now under consideration, passes out 

 of the county north of Newtown township. 



The trap rock throughout this region is much finer grained and more compact 

 than the same rock found in and near Upper Darby. Mr. Rogers, in his report of 

 the State survey, notices a very large dyke commencing some distance west of 

 the Schuylkill, crossing that river at Conshohocken, and terminating in Delaware 

 county " near the road leading from the Lancaster Turnpike to the King of 

 Prussia village." The author has examined this dyke at Conshohocken and at 

 the Gulf Mills in Upper Merion, where there is a good exposure of it, and he 

 labors under a great mistake if he has not seen the same dyke, or very large 

 branches from it, much further southwest than the point designated as its west- 

 ern termination. Be this as it may, such dykes with innumerable smaller 

 branches exist, and that they have been the chief instruments in breaking up 

 the strata of this district of country, and in giving the rock its present meta- 

 morphic character, cannot admit of a doubt. My friend, the late John Evans, of 

 Radnor, before the publication of the State survey, held the opinion that the 

 trap dykes extending to the southern part of that township, (some of which 

 passed near his residence.) were branches of the main dvke passing the Gulf 

 Mills. 



This trap is of a remarkably fine compact texture, especially in the neighbor- 

 hood of Siterville, where it breaks with a purely conchoidal fracture. 



The gneiss of this district, where it has not been too much altered by the trap 

 dykes, does not differ materially from the gneiss of Upper Darby already noticed, 

 although none has been noticed so perfectly porphyritic. Mr. Rogers says the 

 prevailing varieties are, '-first, a massive feldspathic gneiss, some of it'mica- 

 ceous, and some of it like a stratified syenite; and, secondly, a dark, hard, 

 hornblende feldspar gneiss, thinly laminated and strongly striped when viewed 

 in transverse section," Mr. Rogers thinks he has discovered " a remarkable 

 feature in the uppermost or northern bands of gneiss * * * ■* which next 

 adjoin the base of the primal series, in the possession of a less than usual com- 

 pleteness of crystallization in the constituent minerals." Mr. R. has acknow- 

 ledged the difficulty of tracing the dividing line between the two formations, and 

 as he had a theory to support, which this imperfect crystallization favors, it may 

 be possible that his specimens were obtained on the wrong side ofthe dividing 

 line. 



Besides the varieties of gneiss mentioned by Mr. Rogers, as occurring in this 

 district, there is one, not noticed by him, and not found in the southern or first 

 described district. Its peculiarity consists in its quartz possessing a light blue 

 color. This occurs in a belt of considerable breadth, which the author has 

 traced from the eastern part of Radnor to the eastern part of Willistowu. in 



