424 ME. J. DURHAM ON THE VOLCANIC EOCKS 



the fine dust from the top of the contents of the cavity, the central 

 mass, though pure white, is found to be identical in form with the 

 dacite glass in the neighbouring block, but it crumbles into powder 

 shortly after being lifted out. This ^Droves, if proof were necessary, 

 that the white powder is just the last stage of decay of the glassy 

 rock, and that we must therefore conclude that at one time all the 

 cavities in the dacite containing it were filled with the glass. 



About two hundred yards behind the dacite-breccia, a railway- 

 cutting exposes a large section of altered basalt (melaphyre). This 

 section is peculiar as showing the basalt in a much more massive 

 form than is to be seen anywhere else in this part of the country ; 

 it seems neither to be a dyke nor a volcanic " neck," but is more 

 like part of a great lava-fiow. This view receives valuable support 

 from Professor Judd's microscopical examination of two specimens 

 taken from this section ; one of these proves to be a " porphyritic 

 melaphyre " (altered basalt), and the other '' either a volcanic breccia 

 or the vesicular portion of a lava- stream which has caught up 

 fragments of other lavas," in either case indicating a subaerial 

 eruption. 



On the beach immediately to the west of the dacite-breccias, a mass 

 of rock, similar to that exposed in the railway-cutting, forms part 

 of the cliff. Between this mass of basalt and the fault to the west- 

 ward of the Tay Bridge, the cliff is formed of a series of well-stratified 

 beds of similar mineral constituents to those of the dacite-breccia. 

 In these beds the large constituents of the breccia are represented 

 by microscopic fragments which, having been deposited in the waters 

 of the Old-lied -Sandstone sea, form sandstones almost entirely com- 

 posed of comminuted dacite and volcanic ash. Some of these beds 

 would seem to indicate that these volcanic materials had been de- 

 posited in comparatively shallow water, as the well-stratified beds 

 frequently pass up into others that have little or no trace of strati- 

 fication. The dip oi' these beds is well marked, and is, as might be 

 expected, the usual south-east dip of the Old Red Sandstone on this 

 side of the great anticlinal. 



Taking into consideration the presence of altered basalt, both in 

 the railway -cutting immediately to the south of the breccia, and 

 between it and the related stratified beds to the westward, as well 

 as the fact that all the fragments of andesite, of whatever size, are 

 sharply angular, I think we are justified in coming to the conclusion 

 that, between the faults at Scroggieside and Tay Bridge, we have the 

 remains of a basaltic eruption in the immediate proximity, if not 

 actually at the spot itself. This volcano had burst through the quartz- 

 andesite lava of the earlier eruption, and blown fragments of it far 

 and near all over the land and into the neighbouring sea, where 

 some of the larger pieces got mixed up with the shingle of the shore 

 and the finer materials were stratified beneath its waters, the huge 

 masses of the breccia being deposited on the land in close proximity 

 to the crater. Like some of the recent volcanos of the Eifel, little 

 else than pieces of the rock through which it burst seems to have 

 been ejected by it. 



