Sunday, September 30, 2012

John White and a Rock Pile

Actually we  were trespassing as we peered through a pile of rocks dumped just a few feet off a lonely country road in York County, Pennsylvania near Delta. John White and I  had  tried unsuccessfully to locate the landowner. In the process, we learned that the a local power company had dumped the rocks there in conjunction with a project that never happened. Hopefully, the landowner wouldn't mind.

John had expressed interest in these rocks over the past year and had encouraged me to check them out. He had described the material as "just serpentine rock."  For reasons that hindsight now renders lame,  I had not yet done so. What was wrong with me? When a past Curator-in-Charge of the national mineral collection who also founded Mineralogical Record gets excited about a site less than an hour's drive away, it's worth a look.

Then last week, John emailed me an invite to see some of the rocks he'd purchased at the recent Denver show. Attached was the image shown at left. It pictures cabs from material John had found when he first discovered the aforementioned rock pile last year.  His email suggested that after looking at his Denver acquisitions, we drive there.

We did and ended up crawling over the pile of serpentinite rocks. There were numerous pieces worthy of John's cabs. Antigorite was abundant, especially  the columnar picrolite variety. Colors ranged from  very light green to very dark green. Gemmy coatings of medium to dark green  antigorite occasionally comprised the rocks' surfaces. This antigorite was slightly translucent, but less so than antigorite (var.) williamsite.  It also lacked those chromite spots so definitive of williamsite. The  rock beneath the antigorite was grainy, dull greenish gray and black, possibly chrome-bearing, and iron-bearing almost for sure. Though we didn't have magnets, John had learned  when the cabs from his earlier find were made  that much of the black material here was magnetic.

One specimen that particularly interested me is the quartz (var.) prase shown at right. Is presence along with the serpentinite strongly suggests that the rocks on this pile were from the neighboring Cedar Hill or Penn-Maryland Quarries just a few miles east in Lancaster Couty at Fulton Township. Both quarries exploit the same kind of serpentinite rock that's part of the Baltimore Mafic Complex.

 Another piece that impressed me features an intersection of massive magnetite with pale green antigorite (var.) picrolite. Were it chromite rather than magnetite meeting that picrolite, I suspect some williamsite would be gracing the picture as well. Assuming that the rocks originated at either the Cedar Hill or Penn- Maryland Quarries, such a find would seem reasonable. Chromite as well as magnetite is known to occur at both localities

All told, we spent less than an hour at the rock pile. We departed because we were hungry and still marginally clean enough to be served a hearty lunch at a family restaurant a few miles away.

 Thereafter, we continued about ten miles further on  to check out ---with permission---the edges of another rock pile that John knew about. It consisted mostly of  "Wissahickon schist."  We quickly uncovered some graphite, pyrite, and siderite.  This was a much bigger pile of rocks. Unfortunately,  later commitments limited the amount of time we had to see as much of it as we would have liked. Stay tuned.