CARBONIFEROUS PERIOD. 325 



^ith the shales, sandstones, conglomerates, and limestones. But 

 they are thin, compared with the accumulation of rock-strata. 

 The Coal measures contain, generally, 50 feet or more of beds of 

 rock to one foot of coal. 



Iron-ore beds also occur, making other thin layers in the series, 

 and rendering the Coal regions the best iron regions of the globe. 



The following section is an example of the alternations (beginning below) : — 



1. Sandstone and conglomerate beds 120 feet. 



2. COAL 6 « 



3. Fine-grained shaly sandstone 50 " 



4. Siliceous iron-ore 1£ " 



5. Argillaceous sandstone 75 " 



6. COAL, upper 4 feet shale, with fossil plants, and below a thin 



clayey layer 7 " 



7. Sandstone 80 " 



8. Iron-Ore....'. 1 " 



9. Argillaceous shale 80 " 



10. Limestone (oolitic), containing Producti, Crinoids, etc 11 " 



11. Iron-Ore, with many fossil shells 3 a 



12. Coarse sandstone, containing trunks of trees 25 " 



13. COAL, lying on 1 foot slaty shale with fossil plants 5 " 



14. Coarse sandstone 12 " 



The alternations are thus various, and may follow any order. 

 The shales, sandstones, conglomerates, and limestones resemble 

 the corresponding rocks of other periods, and they are distinguished 

 as belonging to the Coal measures only by the fossil plants or animal relics 

 they may contain. Disastrous errors are often made when this rule is 

 not regarded. 



The beds, even when thick, whether of coal or of any of the 

 rocks mentioned, have in some districts a limited lateral extent; 

 yet in this respect the Coal measures differ little from earlier forma- 

 tions. Some of the larger beds of coal are supposed to spread con- 

 tinuously over many thousand square miles of area. 



In connection with the Coal measures of Rhode Island there are extensive 

 beds of quartzose conglomerate, which outcrop at Newport and elsewhere, and 

 form a bold feature in the landscape at " Purgatory/' 2J miles east of Newport. 

 They occur also in Massachusetts, between this region and Boston, showing 

 well about Roxbury. The exact position of the beds in the series is not 

 known, as the rocks have undergone great disturbance, and in some places so 

 much metamorphism that the cementing material is a talcose schist. At Taun- 

 ton, Mass., its pebbles have occasionally been found to contain Lingulae of the 

 Potsdam sandstone {Lingula prima), proving that they are pebbles of this Pri- 

 mordial rock: but whence derived is unknown. 



