A guide to Belfast and the counties of Down & Antrim . Some of the sheetsare compact and massive black basalt; others are vesicularand amygdaloidal, often containing in their cavities zeolitesand other secondary products. The zeolites are frequentlydrawn out in the direction of the flow, indicating a consider-able degree of movement during the formation of thevesicles. Other flows are traversed by a number of verticaljoints, and approach to rudely columnar basalt. The flowsdiffer in the degree of crystallization of the various minerals,indicating different rates of cooling and different condit
A guide to Belfast and the counties of Down & Antrim . Some of the sheetsare compact and massive black basalt; others are vesicularand amygdaloidal, often containing in their cavities zeolitesand other secondary products. The zeolites are frequentlydrawn out in the direction of the flow, indicating a consider-able degree of movement during the formation of thevesicles. Other flows are traversed by a number of verticaljoints, and approach to rudely columnar basalt. The flowsdiffer in the degree of crystallization of the various minerals,indicating different rates of cooling and different conditionsunder which the magma cooled. Prequently one flow ismarked off from the next by a band of red ochre, theremains of a soil weathered from the surface of the older I04 Guide to Belfast. sheet. At Ballygally Head the great height of the rudecolumns known as cornsacks indicates a considerablethickness or depth in the sheet, and the coarse textureof the rock accompanying this proves how variable inthickness were the various flows. At Pleaskin Head and. THE GOBBINS, ISLAND Flows, with Intermediate Bole Beds. Igneous Rocks. 105 the Causeway the cokimnar structure is remarkably developedin the lower part of a flow, while the upper part, which hascooled more rapidly, shows the characteristic starchystructure. Spheroidal structure is common, and whenweather has attacked the surface it brings out the structureknown as onion structure. As the same structures are common to the Upper andLower basalts, and as there is no fixed lithological differencebetween the Upper and Lower sheets, it is impossible todetermine to which group an isolated specimen may belong:it is only by tracing the field relationships of the rocks thata determination can be relied upon. The various degrees of crystallization allow of distinguish-ing the rocks into coarse dolerite or gabbro; true basalt;and on the margins of dykes or on the under surfaces offlows, as at the Rathkenny mine, the glassy type (
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