256 Geological Systems — Geological Maps, ^c. 



as much at a loss to conceive where it begins, as I am to coy- 

 lecture where it is to end. It seems to be composed in part 

 of Werner's alluvial and secondary, but the limits to which it 

 is confined appear to me undefined and speculative, resting 

 on the distinction between depositions accumulating on the 

 bottom of deep water and those rolled and left on the borders 

 of either a sea or lake — a distinction difiicult to make at this 

 distance of time from the operation. 



If sand or rolled masses are the criterion, all the transition 

 puddings and sand-stone would become tertiary. I am un- 

 able at present to investigate the grounds of those speculative 

 opinions, and no reasoning but that elicited by the stroke of 

 the hammer can throw light on the subject. 



CHATOYANT FELDSPAR. 



Since I arrived in this city, I am induced, on inspecting 

 specimens of the feldspar in granite called moon-stone, and 

 of the Labrador feldspar, boih of which present changeable 

 colours, to conclude that the phenomena are, in both cases, 

 caused by the infiltration of water under crystalline layers 

 which form the surface, and 1 am confirmed m that opinion 

 by finding in the river below Shafhausen a granite pebble„ 

 the feldspar of which had the same properties as the Labra-^ 

 dor feldspar. 



