30 ANNUAL ADDRESS, MDCCCLXXIII. 



The next system is the Cambrian, which although con- 

 taining fossils believed but a short time ago to represent 

 the oldest life forms, or primordial types, has but little 

 limestone. It is not of more interest to my inquiry than 

 to note that it contains, in the Huronian series, an un- 

 fossiliferous limestone 300 feet thick. 



The Silurian system, so named by the late Sir Roderick 

 Murchison, is next in order, and it is in it that we first 

 realize how vast a part in nature is played by lime, or 

 to speak more strictly calcium, which is the chemical 

 name for that elementary substance of which lime is an 

 oxide. This Silurian system is well developed in Wales 

 and Shropshire. It took its name from an old British 

 tribe, the Silures (Jukes's Manual of Geology, page 

 530) who inhabited part of South Wales. The thickness 

 of the various beds in this formation is estimated at about 

 6,000 feet. Its characteristic fossils are trilobites, which 

 are found in great number in some of the beds of the 

 Dudley, Wenlock, and Ludlow groups. I am indebted to 

 my friend Mr. Hollier, of Dudley, who is an authority of 

 high repute on Silurian fossils, for some beautiful speci- 

 mens of trilobites, kindly sent for this meeting. On one 

 slab may be counted no less than 28 portions of trilo- 

 bites, besides many other organisms crowded together in 

 wild profusion and in the stillness of death. He has 

 also sent the series of Silurian limestones that lie on the 

 table : they show the variations of the rock and its ac- 

 companying shale. There are about 500 species of fossil 

 shells and organisms in the Ludlow group, in which acti- 

 nozoa, brachiopoda, echinodermata and Crustacea abound, 

 as well as the remains of fishes. It is in this formation 

 that the oldest known fishes have been found in shale 

 below the Aymestry limestone. Thus from this period of 



