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On Composite Lava Flows.

Published online by Cambridge University Press:  01 May 2009

W. Q. Kennedy
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Extract

For many years composite minor intrusions, both sills and dykes, have been known from various parts of the world and most petrologists must have speculated as to the probable effect produced in the event of such composite intrusions having reached the surface in the form of an effusion. For obvious reasons it has not been found possible to trace a composite dyke upwards into a lava flow. However, during the revision of 1 inch Sheet 30 (Renfrewshire) for the Geological Survey, the author encountered, in the neighbourhood of Inverkip, a small village on the Firth of Clyde south of Greenock, certain peculiar lava flows which are believed to represent the effusive equivalents of composite minor intrusions. These “composite lavas”, which form the main subject of the present paper, are of Lower Carboniferous age (Calciferous Sandstone Series) and occur interbedded among the more normal flows towards the base of the volcanic group. Two distinct rock varieties, one highly porphyritic, with large phenocrysts (up to 1·5 cms. long) of basic plagioclase, and the other non-porphyritic, are associated within the same flow. The porphyritic type always forms the upper part of the flow and overlies the non-porphyritic; the junction shows unmistakable evidence that both were in a fluid state along their mutual contact at the time of emplacement.

Type
Original Articles
Information
Geological Magazine , Volume 68 , Issue 4 , April 1931 , pp. 166 - 181
Copyright
Copyright © Cambridge University Press 1931

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References

page 166 note 1 The term “composite intrusion” is here used in the sense given to it by Bailey, and Thomas, in “The Tertiary and Post-Tertiary Geology of Mull, etc.” Mem. Geol. Surv., 1924, 32. See also p. 178 of this paper.Google Scholar

page 166 note 2 For a general and historical account with references to the older literature, see Harker, A., “The Tertiary Igneous Rocks of Skye” Mem. Geol. Surv., 1904. 197–201.Google Scholar

page 166 note 3 Richey, J. E., Trans. Geol. Soc. Glasgow, xvii, 1927–1928, 247–58.Google Scholar Richey, J. E. and others in “The Geology of North Ayrshire” Mem. Geol. Surv., 1930, 69.Google Scholar

page 166 note 4 Tyrrell, G. W., Trans. Geol. Soc. Glasgow, xiv, 1912, 219–57.Google Scholar

page 166 note 5 Bailey, E. B. and others in “The Geology of the Glasgow District” Mem. Geol. Surv., 1925, 137–40.Google Scholar

page 167 note 1 In the Markle type of basalt, macro-phenocrysts of labradorite-bytownite up to 1·5 cms. long are set in a groundmass of acid labradorite, augite, subordinate olivine, and magnetite. For definitions and a general discussion of the Carboniferous olivine-basalts and mugearites, see MacGregor, A. G., Trans. Geol. Soc. Glasgow, xviii, 1928, 324–60.Google Scholar

page 172 note 1 Harker, A., “Geology of the Small Isles, etc.”: Mem. Geol. Surv., 1908, 131.Google Scholar

page 172 note 2 Washington, H. S., “Petrology of the Hawaiian Islands”: I. Kohala and Mauna Kea: Am. Jour. Sci., v, 1923.Google Scholar

page 172 note 3 This rock being a transitional type does not fit readily into the classification. It is, strictly speaking, a basaltic mugearite with the addition of large phenocysts of basic plagioclase but has been named basalt, as it seemed inadvisable to extend the term “mugearite” to include a rock so rich in phenocrysts of basic plagioclase. It might, however, be termed labradoritephyric basaltic mugearite.

page 173 note 1 Buch, Leopold von, Physicalische Beschreibung der Canarischen Inseln, Berlin, 1825, 229.Google Scholar

page 173 note 2 Darwin, Charles, Geological Observations, 2nd ed., 1875, 132.Google Scholar

page 174 note 1 Op. cit., 132–3.Google Scholar

page 174 note 2 Bailey, E. B. and Thomas, H. H., “The Tertiary and Post-Tertiary Geology of Mull, etc.” Mem. Geol. Surv., 1924, 33.Google Scholar

page 174 note 3 Harker, A., “The Tertiary Igneous Rocks of Skye” Mem. Geol. Surv., 1904, 298.Google Scholar

page 175 note 1 Unfortunately there exists at present on suitable term for a magma containing suspended crystals which under conditions of rapid cooling would give rise to a coarsely porphyritic rock. The term “porphyritic magma” is here used to designate such a type.

page 175 note 2 Tyrrell, G. W., “Geology of the Cumbrae Islands”: Trans. Geol. Soc. Glasgow, xvi, 253.Google Scholar

page 176 note 1 Tyrrell, G. W., Trans. Geol. Soc. Glasgow, xiv, 1912, 226.Google Scholar

page 176 note 2 Harker, A., “The Tertiary Igneous Rocks of Skye”: Mem. Geol. Surv., 1904, 256–70.Google Scholar

page 176 note 3 This was found to be the case at Druim na Criche as well as at Roineval, and the phenomenon is not confined to the latter locality, as stated by Harker. See Harker, A., op. cit., 256.Google Scholar

page 177 note 1 Harker, A., “The Tertiary Igneous Rocks of Skye”: Mem. Geol. Surv., 1904, 259 and 263.Google Scholar

page 178 note 1 Harker, A., “The Tertiary Igneous Rocks of Skye”: Mem. Geol. Surv., 1904, 269.Google Scholar Harker, A., “The Geology of the Small Isles, etc.”: Mem. Geol. Surv., 1908, 129.Google Scholar

page 178 note 2 Bailey, E. B., “The Sgù'r of Eigg”: Geol. Mag., 1914, 303.Google Scholar Bailey, E. B. and others, “The Tertiary and Post-Tertiary Geology of Mull, etc.”: Mem. Geol. Surv., 1924, 95–6.Google Scholar

page 179 note 1 There are three conditions attached to the new definition: (1) Composite intrusions are composed of parts of recognizably different composition; (2) As a complex they chill exteriorly against country-rock; (3) The component parts do not chill against one another—if they do the intrusion is multiple, not composite. See Bailey, E. B. and Thomas, H. H.. in “The Tertiary and Post-Tertiary Geology of Mull, etc.”: Mem. Geol. Surv., 1924, 32.Google Scholar

page 179 note 2 This asymmetry resulting from composite extrusion must not be confused with that produced in an intrusive sheet by gravitative differentiation in situ such as in the peculiar differentiated sill described by Iddings from Electric Peak, Montana. See Iddings, J. P. and others in “The Geology of the Yellowstone National Park”: Monograph 32, U.S.G.S., 2, 1899, 82–4.Google Scholar

page 180 note 1 Washington, H. S., Aurousscau, M., and Mary, G. Keyes, “The Lavas of Etna”: Am. Jour. Sci., xii, 1926, 371–408.CrossRefGoogle Scholar

page 180 note 2 Op. cit., 398.Google Scholar

page 180 note 3 Op. cit., 401.Google Scholar