Cockburn, A. M.

2018-05-22T12:46:35Z

2018-05-22T12:46:35Z

1929

St. Kilda, with its attendant islets and stacs, is the relic of what must once have been a much more extensive igneous complex. no trace of effusive or sedimentary rocks, nor of possible metamorphosed inclusions of such rocks, has been found. Though stratigraphical evidence is wanting, it is possible on lithological grounds to classify the complex as Tertiary in age, the rock suite showing strong affinities with the deeper - seated igneous rocks of other areas within the Brito- Icelandic Tertiary province.

The oldest rocks are a series of olivine-bearing and olivine -free gabbros, by far the most abundant type being an olivine eucrite carrying a small amount of hypersthene. On account of the detached nature of the outcrops, the paucity of inland exposures, and the physical difficulties encountered among the cliff sections, it has not been possible to determine the relations of one gabbro to another. i,ioreover, the absence of a base or summit to the complex prevents the determination of the form taken by the individual intrusions.

The gabbros have been tñtruded, first by rather coarsegrained olivine- and olivine -free dolentes in sheet-like form, and / and later by a series of finer-grained dolerites, one of which, exposed in the Village tray and on the northern cliffs of the main island, is a basic olivine dolerite. The contact of the finer -grained dolerites with one another and with the older gabbros is of a puzzling nature, but they are regarded as forming a sheet-like complex. On their contact with the gabbros to the north of Lt. Kilda they appear to dip north -eastwards. Un the western coast they dip south -westwards for the greater part of their exposure, but at Laiiahrig nan Gall, where they disappear under the eucrite of the W. Coast, they are seen dip- -ping northwards.

The basic rocks are ántruded by three masses of grano- -p jyre. '.she largest, and probably the youngest, of these in- -trusions is a hornblende -bearing granophyre carrying about 74% of silica and forming the northern third of the main island.. Its contact plane dips south -westwards at an average angle of probably about 60°. The two remaining intrusions are sheet -like in form and are exposed in the Glen bay to the north and among the south -western cliffs of the :gun and the principal island. c They dip south -westwards at high angles and have been slightly basïfied as the result of incorporation of basic rock. The in- -trusion of the south -western coast is in the form of twó irregular sheets running parallel to one another and separated only by some twenty or thirty feet. Their contacts with the basic rocks, which they have breccia,ted for a wide area, are very irregular. The finer-grained dolerites have been pierced by a complicated system of net -- veining proceeding from the granophyre intrusions and hybrid rocks have been produced at several points as the result of interaction between the granophyre magma and the sur* -rounding basic rocks.

Crushing has been observed within the gabbros at many points, and two types of crushing have been noted: 1st, crushing, with the production of mylonitic rock without re-crystallisation of the crushed minerals, and 2nd, crushing with the production of a granulitised rock in which re- crystallisation has taken place. The cataclastic phenomena appear to have originated prior to the actual intrusion of the various granophyres, and it is thought that earth -disturbances, perhaps the result of gas ex- -plosions preceding the intrusion of the acid rocks, have pro - -duced lines of weakness along which the granophyres have subsequently been intruded. The granophyre of trie Glen Bay intrusion appears to have been slightly crushed after consolidation.

The majority of the dykes on the main island, the un, and Soay strike 14.U. -- S.E., but on the east coast of Soay, on boreray, and among the northern cliffs of the main island they trend 1a.1'. - S.W. ureat difficulty has been experienced in de_ -terra.; IiinL; the respective ages of the different dykes, but a series of dolerite- basalt and porphyritic litchstone dykes (or t}ieir ?devi.tr. i.f ied representatives) are definitely of Post - granophyre a e and have been found cutting all the other dyke types.

The rounded nature of the inland topography suggests glacial action but in the absence of rock- striae, roches moutonpees, moraines, or other glacial phenomena it is not possible to give any account of the glaciation of St. 1;ilda. boulders of gneiss discovered below the 100 -foot contour line in the Village tray and the presence of garnets in the soil within the village wall probably point to ice transport of material derived perhaps from iarris or gist to the east. Iio trace of a raised beach has been found among the islands.

http://hdl.handle.net/1842/30604

The University of Edinburgh

Annexe Thesis Digitisation Project 2018 Block 19

The geology of St. Kilda

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy