152 GEOLOGY OF THE SECOND DISTRICT. 



calcareous veins at the time of tlieir enclosure in the materials of the rock, it seems to be esta- 

 blished tliat 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 everj^ direction ; and into the fissures thus formed, pure calcareous matter was uililtrated 

 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- 

 ino- 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 difiicully, 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 theoretical 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 dififer, 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 iiave come to the conclusion that it is not mica slate. In composition. 



