EVOLUTION OF LIMESTONE AND DOLOMITE 407 



is not favorable to limestone building in epicontinental seas. 

 The preponderance of elastics now forming on the shallows sur- 

 rounding the lands is such that the sediments within the 100- 

 fathom line are generally spoken of as consisting entirely of muds 

 and sands, although important limestone -building areas are found 

 around Florida, Yucatan, and on the Australian Great Barrier 

 reef. The dominance of elastics seems to be related to climatic 

 conditions and the rejuvenation of streams which has accom- 

 panied the rejuvenation of the lands. But shallow, epicontinental 

 seas in times past have been important areas of limestone deposi- 

 tion when their expanse was greater than now. 



Deposition beyond the 100-fathom line. — -The area of the ocean 

 is estimated by Murray as 143,259,300 square miles. The littoral 

 and shallow-water zones comprise about 10,062,500 square miles, 

 consequently the deep-sea area covers about 133,186,800 square 

 miles. The calcareous deep-sea deposits of terrigenous origin, 

 coral muds, and coral sands have an area of about 2,556,800 

 square miles or about 1.9 per cent of the deep-sea area. Of the 

 pelagic deep-sea calcareous deposits, the globigerina ooze com- 

 prises 49,520 square miles, pteropod ooze 400,000 square miles, 

 or a total of 49,920,000 square miles, 37 per cent of the deep-sea 

 area. The total area of deep-sea calcareous deposits thus constitutes 

 about 39 per cent of the deep-sea area. 



The terrigenous non-calcareous muds have a total area of about 

 16,050,000 square miles, or about n per cent of the deep sea. 

 The pelagic non-calcareous deposits have an area of about 64,670,- 

 000 square miles, approximately 48 per cent of the area of the deep 

 sea, of which red clay represents 51,500,000 square miles and 

 diatom ooze 10,880,000 square miles. In total, the non-calcareous 

 deep-sea deposits cover about 59 per cent of the deep-sea area. 



The content of calcium and magnesium in samples of deep-sea 

 deposits collected by the Challenger expedition has been compiled 

 in Table XVII. 



It shows that calcium is more abundant than magnesium, the 

 ratio of calcium to magnesium being about 13: 1. The report on 

 deep-sea deposits by the Challenger expedition concludes that the 

 average calcium carbonate content of the deep-sea bottom is about 



