LESSONS IN GEOLOGY. 353 



immense period of the earth's history, of which no record can be 

 found. Erosive forces were active; sediments inclosing remains 

 of various forms of life were deposited and changed to rocks, and 

 the whole submerged and covered from view by later rocks. The 

 rocks of that era which we can examine seem to be mere remnants 

 of great continents of rocks that have been cut away by erosion 

 and submerged. They stand between great unknown periods in 

 the history of the earth. Breaks in the geological record are 

 numerous, but perhaps no other is as well marked as this one. 



During the balance of the Silurian age thick beds of rock were 

 deposited over a wide portion of the continent, as the Trenton, 

 Niagara and Galena limestones, the Utica and Niagara shales 

 and the Medina sandstones ; the limestone largely exceeding the 

 others in some regions, so that it is mentioned as the characteristic 

 rock of the age. 



All classes of invertebrate life were abundantly represented, but 

 mollusks were most numerous and varied, so that this period is 

 often called the Age of Mollusks. Some fossil fishes have been 

 discovered in European rocks. More than 10,000 species of fos- 

 sils have been described from the Silurian rocks ; remains of sea- 

 weeds and of plants that were terrestrial, as ground pines and 

 others, some like the yew and pine. 



The area of Silurian rocks extends from Newfoundland to Mis- 

 souri, from Lake Superior to Georgia, but they are thickest in the 

 Appalachian region. In the west the limestones are most abund- 

 ant, while in the Appalachians they only make up about one-half 

 of the beds. The limestones were formed in quiet waters, largely 

 from the shells of mollusks, the stems of crinoids, the framework 

 of corals, etc. The sandstones and shales were generally beach 

 deposits, sand bars, mud flats, etc. Some clayey iron ores and 

 salt deposits indicate the existence of marshes and shallow salt 

 lakes. The frequently alternating beds show many upward and 

 downward movements of the crust. It takes but a few moments 

 to make these statements of the events of this age, but they were 

 the work of millions on millions of years. And in this study we 

 need to keep constantly in mind that geological time units are 



L. S. 23 



