Proceedings. 13 
experienced field-geologist has been often led to form a correct 
opinion as to the age of a deposit from its lithological resem- 
blance to a series of known age in another district. 
(3) In conglomerates, coarse grits, and sandstones, pebbles 
and fragments from older beds of known age are often recog- 
nizable, and they tell us that the rock in which they occur is 
newer than the one from which they have been derived. Con- 
glomeratic rocks are often unfossiliferous, so that any evidence 
as to their age is of great value. 
(4) The fossils contained in a rock are the most useful, and 
in fact the indispensable guide for determining the age of a 
series. William Smith, the father of English geology, was the 
first to point out that strata could be identitied in distant 
localities by the organic remains, and that different strata were 
characterized by fossils more or less peculiar to them. This 
was in 1794; but it was many years before his views were 
accepted, and it is only comparatively recently that the method 
of describing rocks by their contained faunas has been com- 
menced. There is little doubt but that all rocks will in the 
future be so described, and ultimately the systems themselves 
will be determined by the possession of a characteristic fauna. 
It is with the older forms of life that we have to do to-night. 
It has been found that the Lower Paleozoic rocks contain 
three distinct faunas, 
(1st) or Cambrian, or Primordial Fauna, 
(2nd) or Ordovician Fauna, 
(3rd) or Silurian Fauna. 
Although we thus speak of distinct faunas, we must carefully 
bear in mind that there is no break in the succession of life, 
but one fauna gradually merges into the next. 
(1st.) In the Cambrian or Primordial Fauna (first distin- 
guished hy the great geologist Barrande) trilobites preponderate, 
mollusca are rare. These early crustaceans exhibit a looseness 
of structure and a less high organization than those of a later 
period. They are characterized by having many body-segments 
and few tail-segments, and are often eyeless. They are the 
