130 SEDIMENTARY OR STRATIFIED ROCKS, BY WILLIAM D. MACRHERSON. 



slates, formed from fine mud, the sandstones, formed from sand, and the con- 

 glomerates, from gravel. There is thus a regular series of rocks made from 

 the finest clay or mud, to the coarsest of gravel. These have the least value 

 of any rocks in a mineral sense. As a general thing, they contain no min- 

 erals in sufficient quantities to pay for the working. They are sometimes 

 penetrated, or split up, by veins which have valuable mineral deposits, so 

 that miners see a good deal of slate and conglomerate, without actually get- 

 ting much mineral matter from them. 



Slates and sandstones, however, are quite often rich in fossils, which 

 give the past history of animal life on the earth. The particular kind of 

 fossils a piece of slate holds, always determines the relative age of the slate' 

 for every age of the earth has had its own animals. The lower slates and 

 sandstones, for instance in the Cambrian Age, had low forms of animal life, 

 and as the ages went on, and new rocks were piled up, different animals 

 appeared, and the old ones, with a few exceptions, died out. From the Cam- 

 brian to the present age, animals have increased not only in size, but in 

 intelligence and complexity of structure. In the Reptillian Age, and in some 

 instances since then r many animal forms have been vastly larger than any we 

 have now, but the size and quality of the brain has doubtless increased 

 steadily and without a break from the dawn of life to the present time. 



Let us see how slates are made, and the fossils accumulate. We have 

 previously told briefly in Nature Study, how the process was. When the 

 earth had first cooled from a red hot mass, and was covered with water, 

 there doubtless could have been little ^ or no mud or sand for slate or sand- 

 stone, till huge masses of rock had been pushed up out of the ocean by vol- 

 canic or geologic action. Then the waves and elements began to grind the 

 mountain of rock into powder, and all this powder, or rock dust, gradually 

 settled under water. In fact, by the action of the wind or rain, everything 

 on land gradually reaches the ocean, and vice versa, all the mud along the 

 ocean that we see at low tide, comes from the land. When animals, like 

 crabs, star-fishes, and worms that crawl about on the bottom of the ocean, 

 die, their remains settle down into the soft mud, and before they decay, per- 

 fect impressions of themselves are made, which endure for ages. The mud 

 becomes hardened into slate, and the skeletons and shells previously buried 

 in it so many years ago, are to-day scarcely less realistic and beautiful than 

 when they breathed and lived. So simple, yet so wonderful is the story of 

 a fossil. To the geologist the sedimentary rocks are highly interesting, while 

 to the hard, practical man of the world, the chemically formed sedimentary 

 rocks, like marble and coal, and the eruptives, with their commercial values, 

 are of the first importance. 



