BY FRITZ NOETLING, M.A., PH.D., ETC. 93: 



and immediately on tlie other side of the hne the poroel- 

 lanite occurs, which in its turn is fohowed by Diabas. 

 Thoug"h the Hne of contact is not clearly seen, there 

 can be no doubt as to the true relations of Diabas. por- 

 cellanite, and sandstone. The specific gravity of 12 

 specimens is as follows : — ■ 



1. White Porcellanite, Weston's Quarry, Pont- 



ville 2.308. 



2. Wliite Porcellanite, Weston's Quarry, Pont- 



ville . . . . 2.346 



3. Red Porcellanite, Melton Mowbray 2.362 



4. Vvhite Porcellanite, Weston's Quarry, Pont- 



ville 2.382 



5. Grey Porcellanite, Melton iVIowbray 2.500 



6. Grey Porcellanite, Mona Vale 2.506 



7. White Porcellanite, Melton Mowbray . . . . 2.522 



8. Red Porcellanite, Weston's Quarry, Pont- 



ville . 2.558. 



9. Brown Porcellanite, Old Beach 2.566 



10. Pink Porcellanite, Ivempton 2.578 



11. Reddish Grey Porcellanite, Kempton . . . . 2.654 



12. Dark, nearly black Porcellanite, Mona Vale 2.700 



The average specific gravity is 2.498, and the above 

 figures seem to indicate that as a rule the coloured 

 varieties are heavier than the white ones, and that the 

 darker tinges are again heavier than the lighter ones. 

 If these few figures permit of such a conclusion, it seems 

 that the specific gravity increases with the following- 

 scale of colour : — 



Lightest Heaviest 



White . . Grey . . Red . . Brown . . Black 



As the colour is unquestionably dependent on a certain 

 percentage of, iron, this peculiarity explains itself. 



3. BRECCIA. — Under this heading I include all 

 those siliceous rocks used in the manufacture of tro- 

 nattas in which angular fragments are embedded in a 

 linely-grained matrix of different colour. The colour 

 of both fragments and the matrix varies a good deal, 

 but most common is a yellow or brown matrix contain- 

 ing lisfhter-coloured frag-ments. 



