OF THE XORTH-EAST OF FIFE. 423 



pieces of a bright red rock, closel)- resembling some rhyolites, 

 are conspicuous among the darker stones ; this reddish rock is 

 pronounced by Professor Judd to be a quartz-andesite (dacite). 

 Though some of the fragments of this rock are more rounded than 

 others, still, as compared with the water- worn boulders which form by 

 far the greater part of the conglomerate, they are remarkably angular. 



I have not been able to find any trace of the dacite either to the 

 eastward of the fault at Scroggieside, or to the westward of that 

 near the Tay Bridge. 



To the west of the conglomerate, with possibly a slight fault 

 between (the nature of the ground making it difficult to determine the 

 point), is a breccia extending about two hundred yards along the 

 shore ; it is almost entirely composed of dacite, the included blocks 

 varying in size from minute fragments scarcely visible to the naked 

 eye, up to great masses, some of which must weigh several tons, a 

 large proportion of them being from two to three feet in diameter. 

 All the pieces of dacite, of whatever size, are sharply angular, just 

 as they must have been when freshly broken off the parent rock. 

 The matrix of the breccia consists of minute fragments of very dif- 

 ferent volcanic rocks ; among these the dacite is readily recognizable 

 by its bright colour, contrasting with the pale green of the other 

 constituents. These minute fragments are seen with the aid of a 

 lens to be as sharply angular as the great blocks. 



Another notable feature of these dacite blocks is that almost all 

 of them contain hollows or cavities, some of these being very large 

 in proportion to the size of the block. 



An examination of these hollows shows that in most of them 

 traces, and in some instances considerable quantities, of a white 

 powder are present. The origin and nature of this white powder is 

 not at first very apparent, as it is clearly not derived from the 

 decay of the dacite, the sides of the cavities being hard and sharp. 



Some years ago a full explanation of the phenomenon was 

 afforded by the abnormal action of the tidal currents, probably aided 

 by a storm which laid bare a part of the rock on the plane of the 

 beach, which is usually buried under gravel and fallen fragments of 

 rock from the overhanging cliff. This exposed a very large block 

 of dacite some six or eight feet in diameter. In the block was a large 

 horizontal cavity some five feet across ; round the sides of the cavity 

 was seen the usual white powder, but the greater part of it was 

 occupied by a glassy rock, bearing a very close resemblance to the 

 pitchstone-porphyry of the Hebrides, a rock which one is scarcely 

 prepared to find among the ruins of the Lower Old- Red-Sandstone 

 volcanos. However, all the tests that could be applied to it went 

 to show that it was an acid glass, in many ways related to pitch- 

 stone-porphyry. To set doubts at rest, specimens of it were sent 

 to Professor Judd, who kindly submitted it to a rigorous examination 

 and pronounced it to be a quartz-andesite- (dacite-) glass. 



Close to the block containing the dacite-glass- a very similar mass 

 forms part of the face of the cliff. In it a hollow, opening vertically, 

 contains a very large quantity of the white powder. On removing 



Q.J.G.S. No. 167. 2g 



