AUGITIC ROCKS. 87 were afterwards confirmed by Hessel, by deduction from analysis. 1 Mode of Occurrence. Basalts and dolerites occur under three general modes, i. As vertical dykes. 2. As sheets or beds intruded amongst older rocks. 3. As tabular sheets poured over the surface, and forming horizontal or inclined beds, often interstra- tified with volcanic ashes, agglomerates, and bands ( f bole. The following may be adduced as examples: 1. Vertical Dykes. These are extremely numerous ever the North-East of Ireland, the North of England, and the centre of Scotland; traversing rocks of different geological ages, from the Silurian to the Oolitic. They everywhere radiate from the great basaltic region of Antrim and the inner Hebrides; and Professor Geikie has come to the conclusion that they are all referable to the same Tertiary period with the volcanic rocks of this region itself. 3 There are, how ever, in the North and East of Ireland, basaltic dykes of much older date. 3 2. Intrusive Sheets. These are prevalent amongst the Carboniferous rocks of Ayrshire, the Clyde basin, and other parts of Scotland. Sometimes they are unex pectedly met with in mining researches, and are often passed through in coal-pits. At other times, they rise 1 Zirkel, Petrog. ii. 282. 2 Address to Geological Section Brit. Assoc. Dundee, 1867. 3 Hull and Traill, ‘ On the Plutonic and Granitic Rocks of the Mourne Mountains,’ Brit. Assoc. Rep. Edinb. 1871.
