408 GEOLOGY OF DELAWARE COUNTY. 



arbitrary, the reader is referred to the map, where it will be seen that they occupy 

 the largest part of Haverford and Marple townships, a considerable scope of 

 Springfield and of Nether Providence. Within this scope, intrusive veins of 

 granite have effected a considerable change in the character of the rock in seve- 

 ral places. These may be seen near Leedom's Mill on Cobb's Creek, and near 

 Darby Creek, on the Philadelphia and West Chester Plank Road. On the east side 

 of the latter creek, one or two of these coarse granitic veins have been beautifully 

 displayed by the cutting made in grading the hill. What is singular, the strati- 

 fied rock adjoining the veins is very much decomposed, while the veins them- 

 selves have undergone but little change. 



The mica slate is remarkably garnetiferous in some places, while in others 

 very few garnets are to be seen. They may be seen in the greatest abundance 

 east of Haverford Friends' Meeting-honse, towards Cobb's Creek, where, in tfte 

 decomposed mica slate forming the bed of the road from the Catholic Church to 

 the creek, they appear like a dark-colored gravel ; but upon inspection each 

 si)ecimen will be found to be a perfectly crystallized garnet. The mica slate 

 near the foot of the hill, which contains imbedded gai-nets, is a light-colored, 

 flaky variety, but the sandy bed of the road resulting from its decomposition is 

 of a reddish color. This is supposed to be caused by the slow decomposition of 

 the garnets themselves. 



The mica slate is garnetiferous on Darby Creek, Naylor's Run, and in other 

 places where the rock is exposed. The garnets are generally small — from the 

 size of a shot to that of a pea, but not unfrequently those of a larger size are 

 found. 



OccasioHally this rock takes in a little kyanite as a component ingredient ; but 

 this is rare, and when it does occur, the kyanite is of a much lighter color, and 

 in much less proportion than that found under similar circumstances near Darby 

 Creek Ferry. Surface specimens of rock thus constituted may be seen about 

 a half mile south of Haverford post-ofiice. 



Quartz rock so usually accompanies mica slate, in subordinate strata, that I 

 need hardly mention that our district forms no exception to the general rule. 

 Large exposures of this rock in place, are found in Marple township. 



In several localities, the mica slate assumes the crimped and folded condition 

 that forms one of its striking characteristics. This is well exhibited on the hill 

 east of Beatty's Mills ; near Lowne"s Run, on the road from Springfield to Chester, 

 and in several localities in Marple township. 



Mr. Rogers extends this Geological district beyond the Brandywine; but most 

 of the strata composing it lose their micaceous character in proceeding south- 

 ward and westward from Crura Creek, though some of them regain that character 

 again as they approach the first mentioned stream. The map will exhibit ap- 

 proximately the character of the rocks there as elsewhere in the County. It may 

 be mentioned, however, that in the section formed by the Brandywine, some of 

 the strata are highly micaceous, and contain garnets, while others have the 

 character of true gneiss, or hornblende, or feldspathic gneiss. 



In Birmingham township, a small bed of highly crystalline limestone is found 

 resting in a synclinal trough of the gneissic strata. It has been worked, but it 

 lies so deei),and the quarrying of it is so much interfered with by water, that the 

 further working of the quarry has long since been abandoned. Mr. Rogers does 

 not regard this limestone as belonging to his gneissic formation, but includes it 

 in his primal division of Palaezoic rocks. It was known to older Geologists as 

 " Primary Limestone." 



In the southwestern part of the County, the mica in the gneiss rock is dark- 

 colored, and frequently is replaced by hornblende. Some distance up the river 

 there are several trap dykes, from which numerous boulders have been ejected 

 and spread around in such numbers and to such an extent, as to have given rise 

 to the idea that they were brought from a distance, and belonged to a formation 

 known to Geologists as drift. As the presence of these boulders can readily be 

 explained from local geological phenomena, the evidence of which is undoubted, 

 it is unnecessary to resort to any other theory to explain their presence. These 

 boulders are spread over a considerable portion of Bethel, a part of Upper Chi- 



