146 THE OLD RED SANDSTONE. 
According to Fourcroy, a full twelfth part of the weight of 
oak, when dried, is owing to the presence of this almost uni- 
versally diffused metal ; and the proportion in our common 
heaths is still larger. It seems easy to conceive how that, as 
generation after generation withered on these heights, and 
were slowly resolved into a little mossy dust, the minute me- 
tallic particles which they had contained would be carried 
downwards by the rains through the lighter stratum of soil, 
till, reaching the impermeable platform of tenacious clay be- 
neath, they would gradually accumulate there, and at length 
bind its upper layer, as is the nature of ferruginous oxide, 
into a continuous stony crust. Bog iron, and the clay iron- 
stone, so abundant in the Coal Measures, and so extensively 
employed in our iron-works, seem to have owed their accu- 
mulation in layers and nodules to a somewhat similar process, 
through the agency of vegetation. But I digress. 
The rock appears in the course of the Elliot, a few bna- 
dred yards above the pastoral village of Arbirlot. We find 
it uptilted ona mass of claystone amygdaloid, that has here 
raised its broad back to the surface amid the middle shales 
and sandstones of the system. The stream runs over the 
intruded mass; and where the latter terminates, and the 
sandstones lean against it, the waters leap from the harder te 
the softer rock, immediately beside the quiet parish burying- 
ground, in a cascade of some eight or ten feet. From this 
point, for a full mile downwards, we find an almost continuous 
section of the sandstone —stratum leaning against stratum 
—inan angle of about thirty. The portion of the system 
thus exhibited must amount to many hundred yards in vertical 
extent; but as I could discover no data by which to deter- 
mine regarding the space which may intervene between its 
Towest stratum and the still lower beds of Carmylie, I could 
