leneous Origin of some Trap Rocks. 125 
and mineralogical discrimination, would hesitate a moment to say, 
that this trap is a crystalline, and the sandstone under it, a mechanical 
rock ; the sandstone was evidently formed by subsidence, from me- 
chanical suspension in water, and is composed of the accumulated ruins 
of other rocks, or of other forms of mineral matter ; the trap was as evi- 
dently deposited and aggregated, not from mechanical suspension, but 
from a state of chemical mobility. But, a chemical rock, lying upon a 
mechanical one! Greenstone trap* upon sandstone? How came it 
there? This is the knot which it would be desirable to untie, al- 
though it would be much easier to cut it, by saying it was made 
there just as we see; so was the Coliseum at Rome, or the Parthe- 
non at Athens, as we might maintain with just as good reason. 
As I have intimated, in the earlier part of these remarks, it has 
often been stated that the trap rocks produce changes upon other: 
rocks, and other forms of mineral matter, with which they come in 
contact, or in whose vicinity they lie. However doubtful or imagin- 
ary this may be supposed to be in some cases; in the present in- 
stance, the change is so manifest, and even palpable, throughout the 
whole horizontal length of this vast mural front, that it cannot for a 
moment be questioned, whatever may be thought of the cause. For 
the vertical distance of between three and four feet, in the sandstone, 
and of four or five in the trap ; from seven to eight in the whole depth, 
including the two rocks at the junction ; both the trap and the sand- 
stone unfold a record, of which it appears to me scarcely _—- to 
give more than one interpretation. 
Beginning with the sandstone, at the depth of from four to on 
feet below its junction with the trap, it appears perfectly unaltered, 
and possesses all the characters which have been stated in the descrip- 
tion. Between three and four feet below the trap, going upward, the 
sandstone begins to grow firmer; and as we ascend, this firmness in- 
creases ; the gloss disappears; the color grows lighter; the red vanish- 
es, and becomes dark grey—light grey—ash grey, and in some places, 
almost white ; while at the same time, the firmness is much increased, 
so that from being a very soft and tender argillaceous sandstone, easily 
splitting into laminz, it has become hard, and difficult to break, striking 
fire with steel, like an overburnt brick, and its fissile character is almost 
or quite destroyed. We should not, where it approaches the trap, 
* The trap at this place appears to contain but _ esate minerals; principally 
veins and spots of quartz, and geodes of quartz crystals 
