152 
GEOLOGY OF THE SECOND DISTRICT. 
calcareous veins at the time of their enclosure in the materials of the rock, it seems to be esta¬ 
blished that those veins "were formed subsequent to the consolidation of the rock ; for it is not 
an uncommon circumstance to discover a shell traversed by a vein of spar. The most rational 
explanation appears to be, that the rock in drying, or in the process of consolidating, cracked 
in every direction; and into the fissures thus formed, pure calcareous matter was infiltrated 
in sufficient abundance to fill the open space thus produced in drying. 
2. Magnesian Slate. 
The almost endless varieties of rock which appear under the form denominated slate, occa¬ 
sion great perplexity, as well as a diversity of views as it regards their age, among geologists. 
This is particularly the case when the rock is destitute of fossils, and lies in a disturbed 
district, where its relations have been subjected to change. 
The rock now under consideration, is one to which these remarks particularly apply. Hav¬ 
ing no trace of an organism, and having at the same time an aspect intermediate between a 
primary and a fossiliferous rock, we find it difficult to settle down upon opinions which we are 
satisfied with, and which we can consider as well grounded or founded in fact. We may, 
however, arrive at one or two important probable truths in relation to this rock and its asso¬ 
ciates, viz. that they may have belonged to the period just preceding that when the earth 
became a fit abode for animated beings. If this should be found a rational opinion, these rocks 
open an interesting field for study, as in them we have the representatives of an era in the 
earth’s history, formed under conditions approaching those which were required for the well¬ 
being of man. The one immediately preceding this era must have been one of great disturb¬ 
ance to the crust of the earth, and that did not admit of the accumulation of large bodies of 
water. The succeeding era was one of more quiet, and which permitted the existence of 
water, but probably with an elevated temperature and high solvent powers ; and hence the 
rocks formed during this period approach in character to the great mass of the truly primary, 
are more crystalline, and are somewhat removed as a whole from the aqueous deposits of more 
modern periods. Whatever may be our views of these rocks, it seems they must coincide in 
part with the preceding. The inherent difficulty, however, attending the acquisition of truth, 
where the subject is so far removed from modern conditions, will ever cause a diversity of 
opinion in many particulars. This field is too tJaeoretical for my object in view. I shall there¬ 
fore proceed to describe very briefly this rock. 
Some diversity of opinion prevails as regards the composition of the magnesian slate. Some 
geologists, and they are those from whom it is dangerous to differ, consider this as mica slate. 
For this reason I have sometimes been disposed to leave the description of this mass to other 
hands, being little inclined to offer opinions contrary to those who have also carefully investigated 
the subject. Still, circumstances have conspired to overcome these scruples, and to lead me 
along in the course I have taken in regard to them. Now, it requires but a very few words on 
the point whether this rock is a mica slate or not; and I must observe, that failing to find any 
mica at all in the rock, I have come to the conclusion that it is not mica slate. In composition. 
