Vol. 54.] ON THE BALA & OTHER ROCKS OF LAMBAY. 139 



these beds with the andesites is seen, as mentioned above, in the 

 cliff on the north side of Seal Hole. 



The coarse conglomerate is visible at sea-level, a little way to the 

 north-east of this fault-line, and also at the top of the cliff along 

 the coast due east of the Bell Rock.^ 



With the occurrence of a thrust-conglomerate on the mainland 

 immediately opposite at Portraine, the question of the origin of the 

 Lambay Conglomerate is one which naturally arises. That earth- 

 movements have affected the conglomerate and the limestone-bands at 

 Kiln Point is obvious from the faults in the former and the curvature 

 of the latter which are to be seen there ; but the i unction of the 

 conglomerate and the igneous rocks never has the appearance of a 

 thrust-plane. The limestone, which is altered by its contact with 

 the igneous rock, is bent, and the conglomerate is faulted against 

 it ; there is no resemblance, however, between the crushed material 

 along the fault-line and the gradual formation of the conglomerate 

 at Portraine. 



The beds immediately above the conglomerate are not seen, but 

 a very short distance up the hill-side occurs a fine conglomerate of 

 well-rounded pebbles of limestone and andesite sometimes an inch 

 long, in a calcareous matrix, and this is covered by the slates of 

 Heath Hill. None of these show any such crumpling as might be 

 expected if the mass had been thrust over the underlying igneous rocks, 

 and the limestone at its base had been crushed and broken up into 

 the conglomerate. At Portraine the tough grits which have been 

 thrust over the underlying limestones are very obviously curved, and 

 similar, if not more pronounced, curvature might be expected in the 

 Heath Hill slates had they been acted on by similar forces, as they 

 would have been more readily affected. The occurrence of the fine 

 conglomeratic bands above the coarse bed also points to the latter 

 being an ordinary conglomerate, for if it had been laid down 

 close to the shore (as appears probable from the size and angularity 

 of its blocks) when the shore-line had increased its distance from 

 this spot, we should expect the materials brought thither to be 

 smaller and better rounded. 



The constituents of the coarse conglomerate have been men- 

 tioned already, and among them, it will be seen, are lumps of black 

 earthy shale. These contain obscure traces of graptolites, and are 

 absolutely uncrushed ; but if the conglomerate were formed by earth- 

 movements, which had broken up hard limestone-bands and mixed 

 their fragments up with pieces of igneous and other rocks, it would 

 be imagined that blocks of such a soft nature as these black shales 

 would have shown some signs of the stresses to which they must have 

 been subjected. 



The matrix of the conglomerate shows no signs of crushing either 

 in the field or under the microscope, but appears to be of a somewhat 



^ On the very top of Heath Hill occur a large number of well-rounded blocks 

 of various rocks, gneiss, quartzite, t'elstone, andesite, fossiliferous limestone, 

 grit, etc. ; but this deposit does not in any way resemble the Kiln Point Conglo- 

 merate, and is very probably a Glacial drift. 



