192 WALKS AND TALKS. 



talks, I will give you a fair and simple explanation of the 

 famous theory of Evolution. 



But what are Crustaceans ? Aquatic animals covered by a 

 crust which is composed of a series of segments or rings joined 

 by their edges; and having more than eight feet. Of these, 

 Lobsters and Crawfishes are examples. And what are those 

 Trilobites ? They are crustaceans in which the body is divided 

 lengthwise by two grooves, into three lobes the axis running 

 along the middle, and a lateral lobe each side. The Trilobites 

 were very ancient animals. I say " were," because the last of 

 them perished millions of years ago, during the progress of'the 

 Carboniferous Age. Here are the tombs of their remotest an- 

 cestors. Here lie their forms imbedded in these primordial 

 sandstones and slates. 



Let me explain about these sandstones and slates. At 

 Potsdam, in northern New York, and throughout that vicinity, 

 a gray sandstone lies at the surface the same as referred to 

 in Talk XIX. It stretches across the St. Lawrence River, 

 and north-eastward along its valley. It encircles the Adiron- 

 dack highlands. This is the Potsdam Sandstone. Westward 

 it stretches through Canada to the Sault Ste. Marie, and along 

 the south shore of Lake Superior to Keweenaw Point. The 

 "Pictured Rocks" are part of it. Southward from this shore 

 it disappears under the Trenton Limestone. It is the Potsdam 

 Sandstone which forms the lower portion of the high cliffs 

 along the Upper Mississippi River. It is in this sandstone 

 that multitudes of these ancestral Trilobites lie packed away. 

 They have had a hard time, however. They are all in pieces, 

 and it is difficult to get sufficient pieces together to describe 

 any one of the species. The sands from which this sandstone 

 was formed must have beaten to and fro in shallow water, or 

 along some beach for many years. These trilobites are but a 

 few inches in length. 



Besides Trilobites, we find in the Potsdam Sandstone, many 

 remains of little bivalve shells called Lin'gula. This is a genus 

 of Brachiopods, a low class of Molluscs. The name signifies a 

 "little tongue," referring to the shape. This is a remarkable 



