Nelghlonrhood of the Glanfs Causeway, &c, 375 



trises fmni the protection they give the land in those places,' 

 -preventing the sea iroin making the same inroads there it 

 did on the adjacent parts. 



Dr. M*Donald and I examined together the dykes at 

 White iiouse point, four miles from Belfast; several of 

 them arc crowded together, three or four run parallel in an 

 E. S. C. direction at about 150 yards from each other, and 

 are in one place crossed by another at acute angles; several 

 of these dykes, I am told, are traced across the county of 

 Down on the op[X)site side of Belfast lough. 



Though these dykes were so near, yet they dffTcred mate- 

 rially from each other; in many the middle part and the 

 sides were not of the same grain, nor constituted on the 

 same principle; in some we found zeolite in the centre, 

 but not in the sides ; in others the middle part was formed 

 by cutting it across (no doubt into prisms), while the sides 

 were a rude mass studded with coarse round stones, about 

 the size of an eighteen-pound ball ; these last Dr. M'Do- 

 nald assured me he had often broken, and found them 

 composed of concentric spheres, like the pellicles of an 

 onion; some of the dykes were of solid massive prisms 

 laid quite across, while one or two had a longitudinal di- 

 vision running through their middle, as in the second dyke 

 at Fairhead. 



In all, the lines marking the construction of the dykes, 

 whether accurate or faint, were across at right angles to 

 their directions, but the perfection of the workmanship 

 was very different ; and when we attacked them with a light 

 sledge, we found some to crumble, being in a state of de- 

 composition, others resisted our efforts, while some broke 

 into small quadrangdlar prisms, like the dykes at Port 

 Spagna and the Giant's Causeway. 



Dr. McDonald showed me in his cabinet prisms he had 

 taken from a quarry (no doubt a dyke) near Belfast ; they 

 were nine or ten inches long, and entirely composed of 

 triano:ular pyramids of the same length, put together as if 

 to illustrate Prop. 7. lib. 12th Eucl. Elem. I had found 

 two or three small triangular pyramids among the quadran- 

 gular prisms at the Giant's Causeway dyke, but at the 

 Belfast dyke triangular pyramids were the sole eleiiientary 

 figure. 



As the shore in Belfast lough is low, there are but few 

 opportunities of examining the materials that come in con- 

 tact with the basalt dykes ; in fact I noticed but two, stra- 

 tified clay and freestone ; this clay is very plentiful on the 

 ghore and the adjacent country; it is arranged in very thin 

 A a 4 horizontal 



