Hornblende syenite, sill in Durness Limestone

Outcrop

Hornblende syenite, sill in Durness Limestone
Sills are sheets of igneous rock formed when magma forces its way between beds of sedimentary rock. In the centre of the photograph a 3 metre thick sill (the browner-coloured layer weathering into angular blocks) intrudes dark grey Durness Limestone near Inchnadamph, Loch Assynt. The contact is parallel to the bedding in the limestone.

Hand specimen

Hornblende syenite, sill in Durness Limestone
The rock is intermediate in composition (between acid and basic) and consists of roughly equal proportions of dark and light minerals. The dark minerals have a prismatic shape, i.e. like stubby pencils. The grain size is intermediate between a deep-seated, slowly cooled granite or gabbro and a rapidly cooled lava, as might be expected for a small intrusion emplaced not far below the surface. In fact, the grain size is finer in the lower part of the sample than in the rest, suggesting that the sill was not intruded all in one go, but contains different pulses of magma that cooled at different rates.

Thin section

Hornblende syenite, sill in Durness Limestone
The rock is made up of long crystals of brown hornblende (some are cut lengthways, some are seen in cross-section) set in colourless feldspar.

Plane polarized light. Field of view 6 mm across

D.J. Waters, Department of Earth Sciences, May 2003