90 OUTLINE ,.| 1 II LD-GEOLCX.Y PARTI 



faces. At last, by a more prolonged submergence and 

 the clearing of the water, marine forms of life, zoophytes, 

 encrinites, and molluscs, made their way into th< 

 and flourished so long as to form a bed of lim- 

 about three feet thick. Subsequently the sediment 

 returned, and as the water was filled up, new coal-growths 

 sprang up as before. 



The observer will find it sometimes possible, by means 

 of fossil evidence, to prove that strata, apparently in their 

 natural order, have really been turned upside down, M 

 that what seems the top of each stratum is really the 

 bottom. This could be shown if we found in one of 

 these strata, a row of fossils in their positions of growth, 

 but with their lower ends uppermost. Suppose, for 

 example, that one stratum contained many erect stems 

 of trees, and that in every case the roots of these stems 

 branched out freely at the upper end into an overlying 

 stratum, evidently an old soil. We could not, in such a 

 case, come to any other conclusion than that the whole 

 of the rocks had been overturned. Again, instead of a 

 series of land-plants, imagine a number of bunches of 

 coral, with their roots still in the position of growth 

 but turned up to the sky. We could only explain that 

 position by admitting that the rocks must have been 

 inverted. 



4. Geological Horizons. Fossils have often a high 

 importance in affording to the geologist a clue in his 

 endeavour to unravel the geological structure of a region. 

 He may discover, for example, that some particular 

 stratum, marked by the occurrence in it of certain fossils, 

 can be recognised and traced over a considerable breadth 



