…in an Archean schist within the Superior Craton. Same outcrop as the criss-crossing dikes I showed yesterday. We’ve got tourmalines on the plane of foliation in the Setters Schist in Maryland, too, but they aren’t aligned like these Canadian tourmalines; instead the Maryland ones are scattered willy-nilly across the plane of foliation, like pencils on … Read more
On our way up Compton Peak the other day, my field crew spotted some fibrous growths of ice growing up and out of the ground (perpendicular to the surface of the mountain): (Joe’s hand lens for scale.) The fibrous habit made me think of asbestos, and then I wondered whether the different shapes of ice … Read more
Other members of the geoblogosphere have been posting brief image-heavy missives on labradorite over the past 24 hours. Collectively, they remind me that I’ve got a backlog of photos from the Adirondacks of upstate New York to share. Here are a few scans of cut and polished cobbles of the anorthosite from the Adirondack Massif, … Read more
Here’s a piece of vein quartz with a series of interesting “cubist disc” shaped holes in it. I interpret these cavities as external molds of some mineral which has now been physically removed or chemically weathered away. So only its exterior shape remains, and I wonder if that’s enough to identify the mineral that used … Read more
Biotite in granite: Riebeckite: Fast and slow versions of each. Wanted to post these ASAP, so someone can use them for the Geology Word of the Week…
On Monday when I went to Powell’s grave, I noticed a few “boulder with a plaque” gravestones, the most distinctive of which was this lovely chunk of rose quartz: Quartz is a good choice, since it’s very very very stable at Earth surface conditions, and thus will stick around a lot longer than, say, a … Read more
A doubly-terminated quartz crystal (or “Herkimer diamond”) is found on a hike in the Silurian sandstones of Virginia’s Valley & Ridge province.
The Shenandoah Valley of western Virginia records the switch in late Ordovician time from passive margin sedimentation associated with the Sauk and Tippecanoe epeiric seas, to active margin sedimentation associated with the onset of the Taconian Orogeny to the east. Higher up in the stack, a similar pattern is seen: a return to passive margin … Read more
After my cousin Brad caught up with us after our Pinkham Notch roadcut excursion, we started up the trail at Tuckerman Ravine, and then detoured to go via the Lion Head. Immediately, we began to see some freaking awesome metamorphic porphyroblasts. During the Acadian Orogeny (in the late Devonian), the original muds and sands (deep … Read more
A few microscope photos for you, showing close-ups of feldspars in igneous dikes in the Crazy Mountains of Montana… You’ve seen these rocks before, when I posted a few field photos from this area in August 2009. I took these images of feldspar phenocrysts in a hand sample (from a quartz latite dike) with my … Read more