X.—Structure and Igneous Activity in the Creag Strollamus Area of Skye

Synopsis

The Creag Strollamus area lies about three miles to the north-west of Broadford, and presents in miniature all the formational and structural elements of the plutonic region of Skye.

The pre-plutonic substructure of the area consists of Torridonian, largely sandstones and grits, overthrust at low angles upon Cambrian limestones. The thrust plane has been thrown into a number of gentle folds on N.E.-S. W. axes, and has suffered displacement by faults of small throw having the same N.E.-S.W. trend. Owing to later erosion the Torridonian sediments remain as a very thin veneer over much of the area, but thicken rapidly towards the north-west.

Tertiary volcanics, very largely basalt lavas, were extruded upon a surface of considerable relief, but have now been partly removed to reveal large stretches of Torridonian. Subsequent movements included the development of a number of N.W.-S.E. or N.N.W.-S.S.E. faults, usually of small throw, as well as a general warping of the area, so that the volcanics are now inclined north-eastwards at angles around 30 degrees.

Particular attention is directed to the disposition of the sub-volcanic surface in relation to the thrust plane. In a south-easterly direction the erosion surface gradually converges on the thrust plane and ultimately, in the area of the Broadford gabbro, descends beneath it.

Gabbros form a number of masses of widely varying size, but evidently represent sheeted bodies, formerly of greater continuity. Gabbro occurs only in localities where, as considerations of structure show, the former presence of volcanics may reasonably be inferred, the sub-volcanic surface representing the downward limit of gabbro just as it limits the volcanics themselves. Within the gabbro masses internal structures can be recognised which agree with those in the basalts.

Petrographically the close relationship between gabbros and basalts is confirmed by the occurrence of grading contacts and the presence within the gabbros of relic textures and small-scale structures, including amygdales, comparable with those of the basalts. It is therefore concluded that the gabbros developed by the transformation of the pre-existing volcanic rocks.

Just as the downward limit of gabbro in the structure of the area is the sub-volcanic surface, so the granites are confined upwards by this surface and downwards by the thrust plane. Granite occurs in place of Torridonian in the pre-plutonic structure. The variation in habit of the granites from “boss-like” to sheeted in a south-easterly direction is to be correlated with this observation.

Having regard to the preservation of pre-granite structures, often of considerable complexity, it does not appear possible that the granites have been emplaced by the mechanical displacement of Torridonian. Transformation of Torridonian, essentially in situ, is therefore considered the more probable explanation. Support for this view is provided by the nature of the junctions between Torridonian and granite. These are invariably merging and show, at least on a limited scale, that granitisation has indeed occurred.