216 An Essay on Rents: (Szrr. 
below : and what is more remarkable in the lower half of the allu< 
vial clay, they are sometimes filled with spar and the usual contents 
of the rents, and in the upper half, with clay deeply tinged with 
iron; and sometimes opposite the whole height of the alluviak 
matter they are filled with iron tinged clay: in both instances the 
rents are covered with only thin strata of soil. In. the Shropshire 
and Cumberland coal formations I have seen rents so circumstanced 
at the earth’s surface ; and at Lead Hills in Scotland, in company 
with Mr. Martin of that place, I met with two such rents, that are 
situated in‘the north side of the valley and to the west of the 
Susannah vein. Other rents that reach to only a few inches below 
the surface are as wide in the alluvial clay just above the top of the 
hard rock as they are helow ;, but upwards, they increase in width 
in such a ratio that each side deviates from 20° to 30° from a per= 
pendicular line.. Opposite the alluvial matter they contain clay, 
mixed throughout with large cobbles, which last are very numerous 
at the bottom, The contents in these parts appear as if they had 
been washed into the rents. I have seen such rents in Cornwall. 
Rents reaching through the alluvial matter exist most abundantly in 
low and smooth mountainous districts, such as Cornwall and Lead 
Hills, 
The. existence of rents in alluvial. matter, though new to men of 
science, js a very important fact. Jt shows us that the alluvial 
matter must have been formed before these rents ;, otherwise, after 
reaching the surlace of the present rocks, the rents, could not have 
passed through the alluvial matter. It.also shows us that the alluvial 
matter was formed from the matter below, when this matter was 
the least able to resist a disintegrating force: and by it we know 
that the alluvial matter has. not been removed since then. Hence 
the rocks or strata underneath such parts have not been in the least 
wasted) by the elements. 
IJ. On Srratipication. 
I have said that the phenomenon of stratification, in one point of 
view, is.an effect of the unequal contraction of the earth’s matter. 
L will now give my. reasons for this assertion. But perhaps it may 
be previously necessary to give a definition of the term. Stratifica+ 
tion consists in that assemblage of tabular masses, wherein any one 
mass is parallel to that next above, and to.that next below it. A 
formation that is entitled to be called stratified must have this ar- 
rangement of parts every where. According to. this. definition, all, 
or nearly all, the red and white sand-stone, and some of the lime» 
stone formations, are stratified ; but the formations of granite, mica~ 
slate, &c, are not stratified, unless they. lie in, hollows, as they 
sometimes do, on the primitive and unstratified mass. Mountains 
divided, in @ few places, into tabular distinct concretions, have 
sometimes. been called stratified ; but to, possess this structure, they 
must be: every, where divided’ into, tabular masses, which have the 
