CGNSHOHOCKEN PLASTIC CLAYS ' 481 



able resemblance to clay beds in other states. It was a matter of great surprise to 

 the writer to find that there was no description nor even mention of these clays in 

 any of the state geological reports, nor in fact in geological literatui-e anywhere, 

 and this despite the fact that they have had an important economic usage for 50 

 years. 



The clays occur on the hill north of Conshohocken, north of the Schuylkill river, 

 about 13 miles above Philadelphia. The clay has been exploited about the little 

 village of Harmarville, along the pike between Conshohocken and Plymouth, and 

 at several points eastward along the ridge to and beyond Ban-en or Lafayette hill, 

 covering an area several square miles in extent. 



The clay rests upon a blue (in places white and blue) limestone supposed to be 

 of the age of the Trenton period. The limestone outcrops in several places in the 

 proximity of the clay and has been quarried as marble quite extensively. A trap 

 dike crosses the area, and the trap rock is exposed in several places. Iron ore and 

 sand are still more intimately associated with the clay, and both have been mined 

 quite extensively. 



The clay, which is a tough, plastic, refractory one\ lies in inequalities on the 

 limestone. It is quite variable in color and character at different points. One pit 

 may show clay, while another close by will contain nothing but sand, and from 

 the same pit is taken bright red clay, variegated red and gray, white, black, and 

 yellow clays, the colors not regularly banded, but quite irregularly mixed. In 

 some places there are considerable bodies of uniform color, while at other points 

 the colors are quite intricately mixed. 



The variation in color and character of the materials probably may be best shown 

 by comparing the following detailed sections, the location of which is shown on 

 the accompanying plan, drawn to scale, on which the elevation above tide is 

 shown. 



Numbers 1, 2, and 3, the openings farthest southwest, are sand pits from which 

 foundry sand has been mined. The upper part, 2 to 3 feet thick, consists of yellow 

 sand, containing many quartz fragments and fragments of iron ore, which appar- 

 ently rests uncouformably upon a light, yellow-colored, clean, quartz sand with 

 rounded grains, like beach sand. In one opening thin, irregular layers of fine 

 gravel occur in this sand 10 feet and 15 feet from the top along with a few small 

 pockets of white clay. The sand shows false bedding in places, and in one of the 

 openings the sand is a light, feathery, micaceous variety. 



Number 4 is a clay pit worked to a depth varying from 50 to 60 feet. There is 

 at the surface a layer 3 to 6 feet thick of yellow sand, in a few places running to a 

 depth of 15 feet. This is underlain by red variegated clay, a brilliant red clay, a 

 black clay containing much charcoal or lignite, and a dove-colored clay. Both the 

 dove-colored and the black clays turn lighter on exposure. These clays are not 

 regularly bedded, but quite irregular in their occurrence. Thus the red clay runs 

 more than half way down the pit on one side, while on the other the black clay 

 extends nearly to the surface. 



Number 5 is an abandoned iron mine which shows a white clay containing large 

 quantities of iron ore and numerous pebbles and cobblestones — some rounded, 

 some angular, and some subangular — the whole mass resembling in a striking 

 manner beds of glacial drift. This mass of material shows a coarse lamination in 

 places, a lamination strikingly unconformable with a finely laminated sandy clay 

 that underlies it in one place. 



LX VIII— Bull. Geol. Soc. Am., Vol. 10, 1898 



