On Dykes of Marble and Quartz. 201 



in my opinion, produced' by the falling down of a part of the frozen 

 peak of Kuiz, carrying with it all the snow that covered it : the 

 thawing did the rest, because it brought down all the immense mass 

 of decomposed granite which covered the sources of the Lagunilla. 

 Not content with these observations, and the reflections which arose 

 from them, I sent commissioners, who recognized in the same desert 

 the occasion of such a disaster. I do not, however, know as yet all 

 the results of their observations, as I returned to this place on the 

 1st instant. 



" I visited the place from whence the river Lagunilla pours its 

 streams from the mountain into the plain, and I there saw that the 

 deluge had come from a height of 200 yards above the level of the 

 river, spreading itself so as to take in the plain. From this point 

 every thing was converted into a sandy waste ; and with the ex- 

 ception of a 'i^^ birds of prey, which were cruising about in all direc- 

 tions, scarcely a single living creature was to be observed in the 

 extent. On a few isolated spots the inhabitants were to be seen 

 reduced to the last extremities of hunger, thirst, and fatigue. 



" The tobacco is generally destroyed, partly by the inundation, 

 and partly by the terror, which prevented any effectual means being 

 taken to preserve it. Thus I fear the revenue will suffer severely.^' 

 — (Quarterly Journal of the Geological Society, No. 3., p. 412, 



On Dykes of Marble and Quartz, in connection with Plutonic 

 Bocks, on the Upper Wollondilly, in Argyle County, New South 

 Wales. By the Rev. W. B. Clarke, M.A., RG.S. 



The tract of country described by the author in this me- 

 moir is situated not far from Sydney and Port Jackson, the 

 river Wollondilly, whose gorge lays bare the geological 

 structure of the district, taking its rise in latitude 34° 26' S., 

 longitude 149° 23' E., and, after receiving the waters of se- 

 veral streams, running into the Nepean river, and emptying 

 itself into the ocean considerably to the south of Sydney. 

 The stratified rocks traversed by the remarkable defiles 

 through which these rivers flow, belong to the steril upper 

 portions of the carboniferous formation si widely spread in 

 Australia ; and these carboniferous rocks are traceable (with 

 occasional interruptions from basaltic dykes), from t)ie dis- 

 trict in question to the borders of the Illavvarra region, 

 where they present a lofty mural escarpment. 



The Wollondilly, however, from its source to its junction 

 with the Uringalla (except near Towrang), is described by 



