No. 50.] 
349 
accident will more or less assimilate them to each other. "When, how- 
ever, atmospheric agency changes, when temperature is diminished a 
certain amount, then organic laws suffer an infraction and beings fall 
victims to the change. A few, however, are capable of surviving or 
resisting this change, and may continue while the mass is extirpated. ^ 
Very few, if any species of animals of the Trenton limestone, have 
survived the atmospheric changes which occurred between that period 
and the carboniferous limestone. 
Natiu-al associations or groups of rocks cannot be determined by mi- 
neralogical composition or character, for observation proves that nature 
associates sandstone, limestone and slates, and it results mostly from 
the fineness, coarseness and miscibility of particles, or their capability 
of transportation. In the primitive construction of the globe, and in 
the elements constituting the primary masses, we find, the siliceous, cal- 
careous and aluminous earths. There is, therefore, nothing remarkable 
in this association. If these views are correct, wc may avail ourselves 
of the principles contained in them in determining or constituting groups, 
and the order in which the members are to be placed. Slates, accord- 
ingly, usually form the superior bed, and the sandstones the inferior, 
though it may be questioned whether unexceptionable rules can be es- 
tablished. In the sandstones, the inferior portion is composed of much 
larger particles than the upper. In fact it is often a coarse breccia. — 
From this fact it follows, that the cause or causes producing the frac- 
ture of rocks, and also that which transports them, is more violent at the 
commencement than at subsequent periods, while, at the termination 
of a group, the particles are extremely fine, as in the calcareous and 
siliceous slates. 
The Period when, and Causes by which the Northern Rocks 
WERE abraded. 
In former reports I have had occasion to speak of abrasions which 
the northern rocks have suffered in ancient times. Those who have ex- 
amined this subject, know full well that it is difiicult, if not impossible 
to fix the precise period when the causes producing those effects were 
in action. That they were comparatively modern, may in some instan- 
ces be shown to be probable by existing records of facts. Though I 
am of the opinion that abrasions will be found to have been produced 
in different periods; for when I examined the Trenton limestone on 
Lake Champlain, I found a layer of that rock, situated about midway 
