LESSONS IN GEOLOGY. 



73 



this limestone U found in beds alternating with Reams of ooal 



. ones j yet let it bo in win: 

 may,!! ulo geologist will at once pronounce it* name. 



It form* :i -no among the rocks ; a land-mark in 



the geological landscape. It is an excellent building-ston.-, mul 



may bo aeon in Irulund forming walla built with singular 



i f rugroontH of the rock fitted almout like irrogolar 



mosaic work. 



In tho iron districts it in used in the smelting furnaces to 



ri<l tho ore of its silica, with which the limestone forms a 



and so molts out ; or again, the agriculturist burns 



lnno-kiln, and uses the limo on his land; then it ix 



ily iiuurriod for ornamental work, since it takes a good 



09), Arckcmidopora (Fig. 70), Amplanu (Fig. 71), and Syrinyo. 

 pom ( Fig. 1'i). These, as the reader will notice, are widely diCo- 

 runt in their nhapo, hut there are many immediate forms. One of 

 those, the Amjilexui (Fig. 71), is a cap ooraL Milne-Edwards 

 showed that in the palaosoio period (which, if reference be 

 made to the summary of geological strata in Lesson XL, will 

 bo found to contain all the Ufe which is exhibited in a fossil 

 state to the top of the Permian peri 3d), the corals bore a simple 

 and striking relation to all those of the same kind which were 

 produced in the neozoic period. 



A longitudinal and a horizontal section of the palaeozoic 

 and neozoic cap corals are shown in Figs. 73 a, b, and 74 a, b. 

 In l-'itf. 73 a, b, aro represented those sections of the 



polish, and the stems of the encrinites and other fossila 

 lend an appearance to the stone which jives it a commercial 

 value. 



So fine-grained a limestone is an admirable material for pre- 

 serving the records of tho past life of the globe. The fossila 

 of this member of the carboniferous system are numerous 

 and perfect : since sometimes the whole of the rock ia one mass 

 of corals and encrinites, showing its true marine character, 

 we shall enumei'ato these first. 



The corals are tho cells which a soft, polyp built round itself ; 

 the creature has tho power of extracting from the waters of 

 the sea the carbonate of lime which they contain. These cells 

 were arrayed round a common axis, in a certain symmetrical 

 form, or sometimes they were built one on the other in 

 defiance of any peculiar order, such as the reef corals of our 

 seas. 



Some of the most prominent of these we have illustrated ; 

 namely, Lithostrontion basaltiform (Fig. 68), Aulopora (Fig. 



coral, and upon comparing them with Fig. 74 a, b, at once it will 

 bo seen that the cross sections of each show a different arrange- 

 ment of tho lamellae, or the walla of division ; for while in Fig. 

 73 6 they are in f oura and multiplea of 4, in Fig. 74 b the govern- 

 ing number is 6. While the other sections show a difference, the 

 palaeozoic has transversal plates, the neozoic has none. This 

 alteration in the form of tho structure of the coral at the 

 change of thia period of life is peculiar. 



The Encrinites aro the distinguishing feature of the moun- 

 tain limestone ; they are, in fact, star-fish on stems, only the 

 rays of the star-fish arc greatly increased. They present tho 

 intermediate stop between tho free and fixed Crinoidea. They 

 have all passed away save two species, which are occasionally 

 found, but are very rare, the surviving members of a vast 

 race. Such a great development did the encrinites reach 

 in the carboniferous period, that we find masses of limestone 

 wholly composed of tho stems of the stone-lilies, as they are 

 rather poetically called. Thia stone ia often seen polished in 



