144 THE STORY OF THE EARTH. 



re-trial Pin beck series. There is nothing to show 

 that the terrestrial surface was more or less than 

 an enlargement of the land which in previous ages 

 of secondary time had supplied the insects, mam- 

 mals, plants and terrestrial reptiles which are 

 found in various beds of the Upper and Lower 

 Oolites, and which, by its varying elevation, 

 had effected the distribution and nature of the 

 sediments, all through the lower secondary pe- 

 riod. 



The limestone beds appear to have owed their 

 existence largely to the influence of the Carbon- 

 iferous limestone, for many evidences are found 

 that it was repeatedly denuded, while the sand- 

 stones and clays were supplied partly by sedi- 

 ments derived from denudations of older slatey 

 and schistose rocks. The existence of the Pur- 

 beck beds, which in Dorsetshire are alternations 

 of thin-bedded white limestone, with bands of 



dark-coloured clay, show^ that the streams of 

 fresh water coming from off the land into the 



lake, charged the fre^h water with carbonate of 

 lime, just as they had previously charged the 



sea; while the uplifting of the earth's surface 

 above the seadevel was SO gradual that there is 

 no break between the marine and fresh water 

 beds. Near the base Of the Purbeck beds in 



Dorsetshire an old land surface is found, known 



to the quarrymed as the Dirt bed. It is well 



sc-n in the cliffs of Lulworth Cove in the Isle of 



Purbe< k, and m the Isle of Portland. Here 



niferous trees, sometimes a foot or two in diame- 



31 rate, but the roots and lower parts 

 of the trunk remain erect as they grew. Some 

 of the coniferous trees show nearly 200 rings of 

 annual growth and a length of sixty feet, Am 



