. Manual of the geology of Ireland. ularia ; but other highly competentjudges think they are probably polyzoan. He mentionedalso annelid tracks and burrows, Histioderma Hibernicum,Arenicolites didymus and sparstis, and Hatcghtoniapcecila ;he also figures tracks which he believed to be molluscan. (jn the section exhibited in the Bray Head sea cliff, thereis apparently a great thickness of the rocks displayed ; thebeds, however, are in some places inverted, and the sectionis made up of a number of sharp synclinal and anticlinalcurves ; jso that the real thickness of the rocks is much less H Geol
. Manual of the geology of Ireland. ularia ; but other highly competentjudges think they are probably polyzoan. He mentionedalso annelid tracks and burrows, Histioderma Hibernicum,Arenicolites didymus and sparstis, and Hatcghtoniapcecila ;he also figures tracks which he believed to be molluscan. (jn the section exhibited in the Bray Head sea cliff, thereis apparently a great thickness of the rocks displayed ; thebeds, however, are in some places inverted, and the sectionis made up of a number of sharp synclinal and anticlinalcurves ; jso that the real thickness of the rocks is much less H Geology of Ireland. than what it might appear to be at first sight from the sec-tion ; Jukes and Du Noyer estimated their thickness to bebetween 3,000 and 4,000 feet. Associated with the Cambrian rocks of this group atBray Head, the Great and Little Sugar Loaves, Carrick-gollogan Hill, and elsewhere, there are great reefs and pro-trusions of quartz-rock. These have been shown, by thelate Mr. John Kelly, to be intrusive, or else to have. Fig. 2.—©Idhamia radiata (Dr. Kinahan, Trans. ). been produced under circumstances different from thoseunder which the associated rocks were formed ; the natureand probable origin of quartz-rock will be considered inChapter XII., Section II., among the other Eruptive rocks. The actual junction of the Cambrian rocks with theCambro-Silurians is not anywhere seen. In the Darglevalley, at the confluence of the Cookstown with the Darglerivers, black Cambro-Silurian shales, dipping N. at 400, arefound close to the Cambrians • but the actual contact ofthe two is not visible. At Greystones, south of Bray Head, the rocks are very Cambrian. 15 similar to those near Bray, and contain similar SW., between Newtownmountkennedy and theRoundwood reservoir, a hard massive quartzose grit,similar to that at Bray Head, was passed through whenmaking the tunnel for the Vartry Waterworks. Somemiles south-east of Roundwood, in the conspicuous out
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