JURASSIC PERIOD. 425 



coral shores a kind of sand is formed by the action of waves on frag- 

 ments of coral and shells. These fragments are rounded by attrition ; 

 then often enlarged by deposit of concentric layers of lime from the 

 saturated sea- water ; and, finally, cemented by the same into hard rock. 

 In some such way oolitic limestones have been formed in all geological 

 periods. We conclude, therefore, that in Jurassic times great coral 

 reefs existed where England now stands. In the Jura Mountains it is 

 believed that the remains of fossil circular reefs or atolls of this period 

 are still detectable (Heer). 



Jurassic Coal-Measures. — In the Jurassic times we have reproduced 

 on a large scale the conditions favorable to luxuriant growth of plants, 

 and for their accumulation and preservation in the form of coal. 

 Hence in many countries we have Jurassic coal-fields. To this period 

 belong the Yorkshire coal of England and the Brora coal of Scotland. 

 To this or the previous period belong the coal-fields of North Carolina 

 and Eastern Virginia, and some of the coal-fields of India* and China. 

 The fine coal-measures of New South Wales, Australia, covering an 

 area of 20,000 square miles, are partly, though not mainly, Jurassic or 

 Triassic, as are also those of South Africa. Jurassic coal-measures have 

 a general structure similar to those of the Carboniferous. Like the 

 latter, they consist of alternations of sands and clays, and occasional 

 limestones, containing seams of coal and beds of iron-ore. The iron- 

 ore too is of the same kind, viz., clay iron-stone. We find here also 

 under-clays, with stumps and roots, and roof-shales filled with leaf- 

 impressions. It is fair to conclude, therefore, that the mode of accu- 

 mulation was similar to that already described, viz., in marshes subject 

 to occasional floods. Jurassic coal, though perhaps inferior as a general 

 rule to Carboniferous, is often of good quality, occurring in thick and 

 profitable seams. 



Dirt-Beds — Fossil Forest-Grounds. — Coal-seams with their under- 

 lying clays are fossil sic amp-grounds ; dirt-beds are fossil soils or forest- 

 grounds. The one graduates insensibly into the other, and both are 

 occasionally found in all strata, from the Devonian ivpward. In the 

 Upper Oolite of England, at the Isle of Portland and elsewhere, there 

 occurs an interesting example of such a fossil forest-ground with the 

 erect stumps and ramifying roots still in situ, though silicified, and 

 the logs, also silicified, still lying on the fossil soil (Figs. 628, 629). It 

 is evident that the sequence of events at this place in Jurassic times 

 was as follows : 1. The place was sea-bottom, and received sediment 

 which consolidated into Portland-stone. 2. After being flooded and 

 covered with river-deposit, it was raised to land and became forest- 



* The plant-beds of India (Gondwana series of Indian geologists) are Permian to Ju- 

 rassic inclusive. — (Manual of Indian Geology p. 102 et seq.) 



