21 8 Dr. Richardson’s Letter on the basaltic Surface 
Additional Evidences. Basaltic Hummocks A 
The arguments on which I have founded my opinions have 
hitherto been all taken from the hollows in our surface, and the 
interruptions in our strata, both which, the concomitant circum- 
stances have led me to consider as so many excavations ; but 
the lofty elevations, and the abrupt prominencies rising sud- 
denly from our surface, when minutely examined, lead us 
irresistibly to the very same conclusion. 
When you and I examined together the line of our northern 
fa£ades, we studiously sought for the points where nature 
had made any change in her materials or their arrangement, 
hoping that at the junctions of these little systems, we should 
find some facts that would throw light on the subject; but 
we generally failed ; want of perpendicularity, or other local 
circumstances, impeding us at the most interesting points. 
On the present occasion she has adopted an opposite line of 
conduct, and in many of the steps she has taken, obtrudes 
upon us demonstration of what she has done. 
Whoever has attended to the exertions of man, when em- 
ployed in altering our present surface, either by levelling 
heights, or by making excavations, must have observed that 
it is the practice of the workmen to leave small, cylindrical 
portions standing, for the purpose of determining the height 
of the old surface, and thereby ascertaining the quantity of 
materials removed. 
To these may be compared the stratified basaltic hum- 
mocks so profusely scattered over our area, and which seem 
to shew how high our quondam surface once reached. 
* Navigators use the word hummock to express circular and elevated mounts, ap- 
pearing at a distance ; I adopt the term from them. 
