A Gneiss Photo

A friend just e-mailed me this great photo of rocks being bullied, with the note:

"A gneiss photo- at least, that's my guess and since it's an obvious pun, I'm sticking to it. The photo is from this site and shows a tunnel being bored beneath the Bronx, NYNY. I've tried to find additional pix of the rock face and the machine, but only came up with a NYC page describing the project."

I responded:

"Actually, I think it's a pile of schist- the Manhattan schist, specifically. The wiki link is not too helpful, but let's see... Oops, you were right:

Deep below the buildings and busy streets of New York City, beneath the labyrinth of subway tunnels and stations, lies the geologic foundation that makes New York City unique in the world. This foundation consists of the city’s five bedrock layers: Fordham gneiss, found primarily in the Bronx;(...)

The Manhattan schist is pretty famous, which is why I jumped to that conclusion. In the photo, since you can't see the mineral grains, I couldn't tell. Remember, the distinction between a schist and a gneiss is the former is mica-rich. In the latter, the T & P are high enough that the micas start to convert to feldspars and other silicate minerals- they lose a lot of the platy nature of schists. The textures could be either one.

So, yes, it is a gneiss photo."

I had not heard of the Fordham gneiss before, at least that I recall, and I guess I'll say that in addition to being a gneiss photo, it's pretty gneiss.