at the Don and Tamar Rivers. 103 
Having been informed of the existence of a bed of gypsum 
in one of these vallies, I visited the place, and found it 
situated in the centre of a plot of good land in a low flat: 
it proved to be a hard, white compact limestone, traceable 
only over a very circumscribed space, and nowhere rising 
above the general level of the surface. There was no 
appearance of stratification; but where exposed the rock 
appeared smoothed and water-worn, disclosing drusy-looking 
cavities, lined with crystallized carbonate oflime. Supposing 
this calcareous bed to be as originally deposited, it would 
seem to have been formed as a chemical precipitate : but it 
is far more probable that it owes its crystalline character 
to the metamorphic agency of heat; and the fact of the 
existence of erupted rock in mass within a few paces lends 
countenance to the opinion. Nor is the probability lessened 
by the consideration that beds of a fine white mazl of 
granular structure, and so highly calcareous as to be capable 
of being burnt into lime, now exist on the flats at Kelso; 
as they are also known to do at Woolnorth, and on some of 
the Islands in Bass’s Strait. Returning to the immediate 
left bank of the main channel of the Tamar, I found the 
greenstone cap of the low rounded eminences, which stretch 
uninterruptedly from Whirlpool Reach nearly to Middle 
Island, descending for the most part to within a few feet of 
high water-mark ; and at one or two places, Point Rapid for 
instance, it goes down en masse under the water: whether 
thus still connected with the massive body of greenstone 
emerging on the opposite side, as doubtless at one time it 
must have been, remains to be settled. Sedimentary deposits, 
in the shape of clay-ironstone beds, clayey and arenaceous 
shales, and beds of clay with carbonaceous intermixture, are 
exposed to a great width, occupying at low water the flat 
shore above ‘‘ Middle Island,” from Ralston’s farm to Wil- 
