DESCRIPTION OF PLATE I. a=anterior end. p-posterior end. b.p. brood pouch. Fig. 1. Beyrichia kloedeni McCoy. Internal cast of left valve (male). ×9. Boocaun, County Galway. Coll. National Museum of Ireland, Dublin. Fig. 2. Beyrichia kloedeni McCoy. Internal cast of right valve (female) showing brood pouch. x 9. Cappacorcogue, County Galway. Coll. Sedgewick Museum, Cambridge. Fig. 3. Ventral view of specimen of Fig. 1. showing profile of valve. Fig. 6. Natural size reproduction of McCoy's figure of specimen. of Fig. 5. Fig. 7. Figure, with outline restored, of specimen of T. R. Jones' figure of B. kloedeni in Ann. and Mag. Nat. Hist., 1855, Pl. VI. Fig. 7. × 9. The specimen is an external mould and should appear hollow in the figure. Fig. 8. An impression in dental wax of the specimen of Fig. 7. × 9. Fig. 9. Reproduction (enlarged) of T. R. Jones' figure. XIV-Note on a Sill in N.W. Donegal (Irish Free State). By GERALD ANDREW, M.Sc. Communicated by Mr. S. H. Straw. DURING field-work on the granite and schistose rocks of N.W. Donegal an example of early basic intrusions (prototectonic sills) was seen retaining ophitic structure. Further search in what were considered likely localities yielded what proved, on slicing, to be unexpectedly fresh specimens. The localities from which the specimens were obtained are as follows (see map on p. 210):-- Geol. Sur. Ireland, 1′′ Sheet 4. Marble Hill Strand (N. 55° 10' 40", W. 75° 53'.) S.E.Shore: Sill dipping S.E. under Quartzite. Specimens 96, 656, 657, 659. Sheet 9. Roadside N. of R.C. Church, on road from Gweedore to Dunlewy (N. 55° 1′ 50′′, W. 8° 8′ 40′′.). Speci men 544. Sheet 4. Roadside, 1/6 mile N. W. of Derrycasson Middle Road from Downies Pier to Doagh Bay. Specimens. 185, 405, 406. : In addition to these specimens, the collection of slides in the possession of the Geological Survey of Ireland, which I have been able to examine by the kind permission of Mr. Hallissy include examples of rocks with original augite preserved. These slides are referred to by the number of the specimen in the Survey Collection, and may be distinguished by the prefix letter I. The specimens were collected more or less casually, as attention has been primarily devoted to the relations of the granite mass to the country rock, and further mapping of the schistose series, it is known, will reveal other types of the prototectonic intrusions. The Survey Collection contains many slides of rocks consisting of a brown hornblende1 distinct 1. Mem. Geol. Sur., Ireland, N. W. Donegal, 1891, p. 140. from that developed in the epidiorite facies, with a small interstitial content of decomposed felspar, and accessory biotite, (I 293, I 307, I 275, all from sheet 10, east of the Granite). The general character of the type collected by the author is indicated in the following notes on the slices. 544. This is the freshest rock, and has the following petrographic characters. Structure. Ophitic. Minerals. Accessory. Apatite, not abundant, enclosed in the colourless minerals. Opaque ore, in irregular thin plates, usually surrounded by a narrow border of pale actinolitic amphibole, pale yellow-pale green; also associated with a strongly pleochroic biotite (opaque in maximum absorption-position); apparently intergrown with the biotite plates. The crystals of ore are moulded on the felspar laths, and pierce the augite. Augite. Brownish variety, colour patchy, and occasionally more intense towards the margin of the crystal. No zonal structure apparent. Bordered peripherally with actinolitic hornblende, like the ore. Associated occasionally with hornblende crystals of deeper colour than the actinolitic variety. The two varieties do not grade into oneanother, and the deeper, and browner, variety appears to be truly "primary." Hornblende. Crystals of hornblende (the deeper, browner, variety) also appear with no visible connection with augite individuals. Biotite. Crystals of deeply pleochroic biotite also appear with no visible connection with the ilmenite plates. Felspar. In idiomorphic laths, with fine pericline twinning and moderately broad albite twinning. Larger individuals commonly zoned. Extinctions (symmetrical about the albite lamella) maximum: inner zone 38°; outer zone 35°. The felspar is therefore a medium labradorite. Small amounts of untwinned felspar occur interstitially, with R.I. less than that of quartz, which is found in small quantities also. The augite occurs in aggregates of variously oriented crystals and is moulded on the plagioclase. No. 659 is also a fairly fresh rock, differing from 544 in the hypidiomorphic structure, the greater abundance of the compact, and in this case, more deeply coloured (green to deep blue-green) hornblende. The felspar is also much more acid, the plagioclase being a medium andesine, and interstitial micrographic quartz-felspar moulded on the plagioclase is a conspicuous feature. Nos. 657 and 656 are considerably decomposed. No. 656, however, retains the fresh augite, and the structure is hypidiomorphic-granular. Chlorite appears as a secondary mineral in association with the ore-mineral. The felspars are turbid. No. 96, the marginal specimen first collected, gave a particularly instructive section. At one end of the slice, a fresh hypidiomorphic, fairly fine-grained rock is to be seen, with rounded crystals of augite, laths of a medium oligoclase, and a small amount of the interstitial micrographic intergrowth of quartz and felspar. Towards the other end of the slice is a fairly sharp passage into the typical decomposed rock with actinolitic hornblende in spongy aggregates of felted needles in a ground-mass of decomposed felspar Shear planes pass across the slice, marked by a thin zone of chlorite. No. 185 is a rock similar to 96 in structure, but the augite alone remains fresh, scattered through the mass of decomposed felspar. Nos. 405 and 406 are completely decomposed, and were collected from within 10 feet of 185. Of the collection of the Geological Survey of Ireland, I 274, (Sheet 10, S.E. of Moyle, on road 1 miles S. of Milford) has fresh augite, with fringing actinolitic hornblende, and the felspar is completely decomposed. In Sheet 4, I 3762, from " mile S.W. of Tamney, at head of Lough," and, across Mulroy Bay, I 424, from "W. of Trabeg, Downies Bay Pier," and I 427, from "on road close to Corn Mill, mile S. of Drumroe" (this locality is probably 1. Mem. Geol. Sur., Ireland, N.W. and Central Donegal, 1891, pp. 145 and 146. 2. Ibid, p. 145. |