A SKETCH OF GEOLOGY. 15 



swept from the face of the earth long before the appearance of man and his 

 contemporaries." The fact that fossils show the gradual development of 

 organic life from its lowest to its highest types, affords to science the means 

 of determining the relative age of the different strata in which they are 

 imbedded ; rocks, for instance, embracing remains and traces of fishes, must 

 be younger than others in which only fossils of shells and corals are contained ; 

 or, again, strata only imbedding seaweeds and other low forms of the vegetable 

 kingdom, must be older than those in which leaves and fruits of the trees of 

 our present forests are found. Fossils tell us by their character, and by 

 their mode of preservation, whether they have lived at the bottom of the 

 ocean, in fresh water lakes or rivers, or on the dry land. If they lived in 

 the seas, they also inform us whether these were quiet or stormy, deep or 

 shallow. By the data deduced from fossils, geologists are able, to-day, to write 

 the geographies of former geological epochs, and to mark oat on maps the 

 exact distribution of land and water prevailing on our globe during those 

 periods. What the compass is to the navigator, guiding him across the 

 pathless ocean into the intended port, are the fossils to the geologist in his 

 scientific investigations, and in his explorations for economical and industrial 

 purposes. They not only tell him the history of the earth, and of its former 

 inhabitants, they also show him the fields where the gold and diamond hunter 

 may find those brilliant treasures with which our daughters like to heighten 

 and brighten their natural charms. They indicate the places where to dig suc- 

 cessfully for the briny water which furnishes our table with the most indis- 

 pensable spice, or for the valuable oil illuminating our> houses and parlors. 

 They also locate the mines from which we extract that invaluable mineral which 

 not only affords us comfort at home, but also speeds our travels across the 

 oceans and the plains. 



Fossils are the letters in the geological alphabet. To read the latter you 

 must know the former. Without their thorough understanding, no successful 

 study of geology is possible. 



GEOLOGICAL PERIODS. 



During the infancy of the geological science, it was generally believed that 

 different periods had existed in former ages, which were separated by sharply 

 defined division lines, and that each period possessed its own creation ; which 

 meant that all the different species of animals and plants of a certain period 

 were created and destroyed during that period, and that, for the succeeding 

 epoch, an entirely new creation had to take place. This belief in the total 

 destruction of all existing life at the end of the different geological periods, 

 called cataclysm, originated from the fact that different groups of strata con- 

 tain different groups of fossils, and was upheld by the belief in a priori ere- 



