The Marys Peak Sill was intruded during the Oligocene, about 30 million years ago. It intruded conformably (and is thus a sill) near the base of the Tyee turbidites, a few hundred feet above the contact of that unit with the underlying Siletz River Volcanics. I find the Marys Peak Sill of particular interest for its magmatic differentiation. Overall, the sill is gabbroic in composition, but it cooled to have components that are intermediate to felsic as well. The fragment above is unusually fine-grained for the sill, and I suspect it may be part of the quenched rim- that is, rock that represents the pre-segregation composition of the the original melt, that cooled nearly instantly as it was injected into the cool host rock.
But of particular interest is the lighter line running horizontally across the upper portion of the fragment. As the magma cooled, the volatile component was excluded from the crystals, and was thus more and more concentrated in the melt- effectively driving gas pressure through the roof... literally. Toward the end of the crystallization, the vapor pressure got high enough to fracture the overlying solid rock, and the very last liquid bit of magma, called aplite, was driven out into the network of fine fractures. I've read these actually extend out into the overlying Tyee, though I've not seen that myself. The example above is fairly typical for what I've seen in the area: subtle, and only a mm or two across.
This was a spur-of-the moment choice when Dana visited in July- I wanted her to see the old growth noble fir forest on the mountain top (that'll be tomorrow's photo), and I've always thought the aplite dikelets were pretty cool. However, I'd never really thought to look for them in an organized way- I'd just find them occasionally hiking up the north side trail. It turns out, if you walk down the trail from the summit parking area, and keep an eye on the cobbles and pebbles of sill weathered out onto the ground, they're not that uncommon or hard to find.