214 Dr. Richardson’s Letter on the basaltic Surface 
for here the strata are so distinctly marked that we know 
each of them when we find it again after any interruption. 
In the rest of our precipices and fapades, the similarity of 
the strata deprives us of this advantage ; yet in their smaller 
interruptions, the eye, by tracing the rectilineal course of the 
strata, and so connecting the separated parts, can establish 
their former continuity : while in the greater intervals we 
must rest our proof on analogy alone. 
That we may be entitled to carry this style of induction 
into the interior of our basaltic area, and to apply the same 
reasoning to enable us to form a similar decision upon the 
more stupendous interruptions of our strata, which I have 
already enumerated, it becomes necessary to explain the geo- 
logical construction of our area, — the strata of which it is 
formed — their arrangement — and their inclinations. 
An extensive limestone stratum, white as chalk, and about 
two hundred feet thick, seems to form the base of the whole 
district I limit myself to : upon this, accumulations of rectilineal 
and parallel basaltic strata, are heaped up to most unequal 
heights. 
This great calcareous stratum seems not to be an accurate 
plane, but rather to resemble a bason, as every where at its 
periphery it dips to the interior ; yet the plane of its section 
has a slight ascent to the southward : a recollection of these 
circumstances will enable us to account for every appearance 
this stratum exhibits, as it happens to be disclosed to us ; and 
by the converse, an attention to these appearances will enable 
us accurately to determine the position of the stratum. 
This stratum, from Bally castle to Solomon’s Porch , (about 
twenty-five miles, ) keeps very nearly the level of the sea. 
