• Abstract and Figures
• Public Full-text

Prof. Hull—On the Structure of Trap-rocks. 153 LOGICAL MAGAZINE, we have reproduced the aboye diagram woodcuts from Mr. E. Ray Lankester's excellent monograph on " The Fishes of the Old Red Sandstones," Pal. Soc. Mon.,vol. xxi., 1868, part i. p. 15.—EDIT. GEOL. MAG.] This opinion has been greatly supported by the late Dr. A. Ktoth (Zeitschrift d. Deutschen Geolg. Gesellsch., 1872, p. 1), who found Pteraspis (or Cyathaspis) in one piece, together with Scaphaspis.1 But I cannot agree with Kiinth, who thinks that Pteraspis belongs to the CRUSTACEA ! nor with Eichwald, who supports the old notion of Kner, that Scaphaspis is the bone of a Cephalopod. Pteraspis Kneri occurs in the uppermost Silurian beds of Galizia, associated with Leperditia, Beyrichia, Tentaculites ornatus and tenuis, Orthonota rotundata, Pterinea retroflexa, etc. It is also very abundant in the Bed Marls and Sandstones covering the Silurian Limestone. These sandstones represent the lowest stratum of the Old Red Sand- stone, and contain, besides Pt. Kneri, remains of Pterygotus and pieces of ornamented tuberculated shields very much resembling those of Pterichihys and Coccosteus. I am satisfied that there exists in Galizia an unbroken succession of Animal life from the Uppermost Silurian strata to the Lowest Devonian, just as is the case in the West of Eng- land, in Herefordshire. (See Murchison's "Siluria," 1867, p. 243.) In perusing the valuable Memoir by Mr. Eay Lankester (in vol. xxi., 1868, and vol. xxiii., 1870, published by the Palaeontographical Society), I was sorry to observe that a former article of mine on the Russian Cephalaspidians (Ueber Thyestes verrucosus iind Cephala$pis Schrenkii, in Verhandlungen der Kaiserlichen Mineralogischen. Gesellschaft, St. Petersburg, 1866) had been overlooked by him. In that article I established the new genus Tremataspis, which, to some extent, connects by its microscopic structure the genera Cephalaspis and Pteraspis. I regret that I have not any separate reprints of that article re- maining, or it would have afforded me much pleasure to forward a copy to the author of " The Fishes of the Old Red Sandstone," but doubtless the " Verhandlungen," etc., exists in the Library of the Geological Society of London, to which I beg leave to refer Mr. °Ray Lankester. III.—ON THE MICROSCOPIC STRUCTURE OP THE LIMERICK CARBONI- FEROUS TRAP-ROCKS (MELAPHTRES). By PROFESSOR EDWARD HULL, M.A., F.R.S. (PLATE VIII.) [Bead before the Royal Geological Society of Ireland, 12 March 1873.] R. MACALISTER, in his able Presidential Address, has furnished D the Society with a summary of the work done, principally by German petrologists, with the object of determining the mineral constitution. and structure of plutonic, metamorphic, volcanic, and other rocks, by the aid of the microscope. Forbes, Sorby, and Allport amongst ourselves have also represented British petrologists in the same field, but perhaps have not been sufficiently forward in giving' 1 For Mr. Lankester's reply to Dr. Kiinth's paper (extracted from the Academy) see infra, p. 190. 154 Prof. Hull—On the Structure of Trap-rocks. the results of their labours to the world. As representing the Geolo- gical Survey in this country, I may mention that I have been fully alive to the great benefits to be derived from the use of the microscope, and nearly one hundred specimens of sliced rocks have already received a certain amount of scrutiny, and the results have been noted. These results will be from time to time inserted in the Explanatory Memoirs which are published along with the Geological Maps; and the Memoir issued last year (1872) on the Geology of portions of Counties Down and Armagh, near Banbridge,1 will be found to contain the first of these published series of observations on the microscopic structure of some of the rocks of the district. Amongst the rocks which I have had opportunities of microsco- pically examining, the basalts and dolerites of the North of Ireland and the older melaphyres of County Limerick have received some attention; and I purpose in this paper to institute a comparison between these two classes of rocks, which may be considered as the representative augitic lavas of the Miocene and Carboniferous periods in Ireland. Authors of Papers on the Limerick IVap-rocks.—The geological structure of the Limerick Carboniferous basin has been described by several writers. Of these may be specially mentioned Mr. Wearver,* Dr. Apjohn,* Mr. Ainsworth,* Sir E. Griffith,5 and Messrs. Jukes, Kinahan, and Wynne.6 From the description, often exceedingly full, which these authors have furnished, it is now understood that the trap-rocks of the Limerick Carboniferous basin are themselves of Car- boniferous age. As Mr. Kinahan observes, these traps and associated ashes form two great deposits, occupying well-defined geological horizons; the lower coming in a little above the base of the Lower Limestone, immediately over the cherty beds that are taken as the division between the Lower and Upper Limestones; and the upper, a little below the basal shales of the Coal-measures.' There seem to have been two principal out-pourings of lava from sub-marine or subaerial vents, accompanied by eruptions of ashes and agglomerates, the materials having been strewed over the adjacent seas ; and Mr. Jukes suggests that some of the detached bosses of trap are to be regarded with great probability as the volcanic foci or " necks" of these old Carboniferous craters.8 Names and Mineral Characters of the Rocks.—Though these trap- rocks present considerable variations in appearance and texture, they are more or less basic (with one exception), and probably augitic. The terms they have received, however, from various authors are 1 Expl. Memoir to accompany Sheet 48 of the Maps of tha Geol. Surrey. * •' On the Geological Relations of the East Coast of Ireland." Trans. Geol. Soc, Lond., vol.,v., by Thomas Weaver, M.K.I. A. * " Observations on the Trap District in the Co. Limerick." Jonrn. Geol. Soc. Dub., vol. i. p. 24 (1832). * A Further Account of the Limerick Trap-rocks, by William Ainsworth, (1833) with Map. 6 Geological Map of Ireland, -with reference (1839). * Explanations of the Geological Survey Maps, Sheets 143, 144 (1860). 7 Ibid. Sheet 44, p. 24. 8 Ibid. Sheet 143, note p. 10. Prof. Hull—On the Structure of Trap-rocks. 155 numerous, such as "Greenstone," "Basalt," "Porphyry," "Amyg- daloid," "Spilites," "Diorite,"and "Melaphyre," Let us hope that the term " Greenstone " will in course of time—and chiefly through the aid of the microscope—fall into oblivion; at present it can only be regarded as a word intended to conceal our ignorance. Assuming the words " greenstone " and " diorite " to be synon- ymous, and intended to designate a crystalline rock composed of hornblende and felspar, it is questionable whether such a rock is actually to be found amongst the Limerick trap series, although crystals of 'hornblende do sometimes occur as accessories. As stated by the officers of the Geological Survey," These rocks are of dark and light green, dark and light blue, or reddish colours, the latter being often due to decomposition. Their texture is generally compact, or fine-grained, with crystals of felspar widely dispersed through the mass; other crystals are rare, and some even of those traps, which from their dark colour have been called ' greenstone,' seem almost entirely felspathic. Some of these rocks pass imper- ceptibly into porphyries and amygdaloids, which have many varieties in the different localities in which they are met with." Mr. Jukes adds in a note, " A detailed ohemical investigation into the compo- sition of these varying trap-rocks would be very interesting and instructive. Without it, it is impossible to guess the proportions in which the felspathio are mingled with the augitic or hornblendic minerals; or even to decide on the nature of the felspars them- selves." l It is exceedingly interesting to observe that similar rooks occur in great force amongst the Lower Carboniferous beds of Scotland, forming the ranges of hills which bound the Coal-field of the Clyde basin along both its northern and southern borders. They have apparently been erupted from volcanic vents at the same, or nearly the same, period of geologic time as the Limerick traps themselves, while in the "toad-stones," or basaltic amygdaloids of Derbyshire, we recognize the English representatives'. Thus it appears that in the regions now occupied by the S. W. of Ireland, the centre of Scot- land, and the centre of England, volcanic craters, in all probability submarine, were in active operation, pouring forth over the bed of a highly populous sea sheets of lava, and vomiting forth ashes and agglomerates amongst forests of crinoids and tribes of molluscs and corallines. When, in 1868, I brought over some specimens of these rocks from Scotland, and showed them to Mr. Jukes, he at once recognized their similarity to " the Limerick trap-rocks." The case here mentioned is an instance of the homogeneous character of igneous rocks of the same age over considerable tracts of country, which seems to me to have important bearings on the question of the internal fluidity of the earth below the solid crust. Microscopic slices.—Of the twelve sliced specimens which I have examined, taken from various places over the district, all but one belong to that general group of augitic rocks which may be con- 1 Explanation to Sheet 44, p. 9. 156 Prof. Hull—On the Structure of Trap-rocks. veniently named " melaphyres,." and -which are the representatives in Mesozoic and Upper Palaeozoic times of the more recent basalts and dolerites. In general structure these Limerick melaphyres consist of a glassy eolouriess felspathio base, in which are imbedded crystalline grains of augite, magnetite (or titano-ferrite), pseudomorphs of.

Load more

  9

Copy Link