392 



THE POPULAR EDUCATOE. 



LESSONS IN GEOLOGY. XXII. 



THE JUEASSIC FORMATION LIAS. 



IMMEDIATELY succeeding the Triassic group in an ascending 

 order is the Jurassic formation. These series of rocks have taken 

 their name from the fact that they are extensively developed 

 in the region of the Jura mountains. In our own country they 

 occupy a marked position, stretching diagonally across the island 

 from Dorsetshire to the north of Yorkshire, in a band which 

 averages in breadth some thirty miles. The Jurassic formation 

 contains two notable mem- 

 bers the Oolite and the 

 Lias. The oolite rocks are 

 peculiar for the manner in 

 which their grains are 

 agglomerated. The rocks 

 appear as if formed of the 

 minute roe of fish, hence 

 their name from the Greek, 

 (aov(o'-on), an egg; but the 

 student must not suppose 

 that this oolitic structure 

 is confined to the rocks 

 of this period. It is here 

 that it finds its chief pro- 

 minence. The lower mem- 

 ber of the group, the lias, 

 is also specially interest- 

 ing for the remarkable evi- 

 dences it contains of rep- 

 tile life. As it meets us 

 as we ascend from the 

 trias, we shall consider it 



first. 



THE LIAS. 



Although there are many 

 reasons why the lias should 

 be elevated into a distinct 

 formation, yet geologists 

 usually class it with the 

 oolite. They appear toge- 

 ther, and generally the 

 strata at their juncture 

 are conformable. There is 

 an example, however, in 

 the Jnra where this is not 

 the case. The name lias 

 is an English provincial 

 name, expressive of the 

 appearance of a lias sec- 

 tion as it catches the eye ; 

 the alternations of grey 

 limestone rock, whose thin 

 bands are separated from 

 each other by dark bands 

 of argillaceous deposit, 

 give a riband appearance 

 to the rocks, or as if layers 

 above layers had been 

 superimposed to accumu- 

 late the beds. 



These peculiar beds are 

 the lowest members of the 

 group, and are known by 

 the name of " lias rock." 

 Clay greatly predominates, 



and the whole complexion of the deposits points to a marine 

 formation carried on during tranquil times, and away even 

 from the commotions of a shore. Throughout the whole of 

 its length there is no appearance of nnconformable strata, 

 but the bands retain their respective positions. This is the 

 more remarkable, since we find many zones distinctly marked by 

 certain ammonites. Other shells pass from one zone into 

 another, gradually dying out, but the ammonite peculiar to the 

 zone almost wholly disappears suddenly. In a time of great 

 disturbance or rapid change, when the sea-bed became elevated 

 aoove the water, or the nature of the deposit altered abruptly, 

 such a change in the life occupying the ocean-bed would not be 



remarkable ; but to find a generation of ammonites becoming 

 extinct, while to all appearance the conditions of their existence 

 remained unchanged, is, to say the least, peculiar. Probably 

 the least objectionable explanation of the fact is, that vast and 

 long pauses occurred during the deposition of the lias strata, 

 such pauses as gave time for a species to die out ; and when the 

 deposition re-commenced, there were very few representatives 

 of the ammonites of the last age, and these smitten with the 

 decay of their species did not revive, but soon submitted to the 

 fossilising process. Although the lias is wanting in violent 



transitions, yet it is well 

 capable of subdivision into 

 Upper Lias, Middle Lias, 

 and Lower Lias. 



The Lower Lias. Su- 

 perincumbent on the bone- 

 bed which caps the trias is 

 the " lias rock," an assem- 

 blage of laminated lime- 

 stones which are parted 

 from each other by the pe- 

 culiar layers of clay which 

 have provided the whole 

 group with its name. Some- 

 times these layers of clay 

 and limestones run toge- 

 ther into one consolidated 

 stratum. The clay is usually 

 devoid of organic remains, 

 while the limestone is fos- 

 siliferous. This lower lias 

 averages from 600 to 900 

 feet in thickness, and has 

 been divided into no less 

 than six zones of life, each 

 marked by its own pecu- 

 liar fossils. 



The Middle Lias is a 

 marlstone series composed 

 of calcareous sandy beds, 

 which contain deposits of 

 iron. The Cleveland iron 

 works, which in the last 

 few years have altered the 

 whole character of that 

 agricultural district, are 

 established on a band of 

 ironstone occurring at the 

 lias, which is often sixteen 

 feet thick, and is said to 

 cover some hundreds of 

 square miles, producing 

 on an average 20,000 to 

 30,000 tons per acre. 



The Upper Lias contains 

 the clay shales largely de- 

 veloped in Yorkshire. It 

 is full of belemnites and 

 ammonites, and is valuable 

 for the quantities of iron 

 pyrites it contains. This 

 mineral, upon certain treat- 

 ment, can be made to give 

 up its sulphur ; but it is 

 chiefly employed in the 

 manufacture of sulphuric 



acid. The same deposit also yields clay, from which alum 

 (sulphate of alumina) is procured. The limestones are used 

 for mortar, and the Yorkshire cliffs in the neighbourhood 

 of Filey are mined if running holes, like rabbit burrows, into 

 the clay can be called mining for calcareous nodules, called 

 septaria, which are ground down and pulverised for hydraulic 

 mortar. The septaria are concretionary nodules, which in their 

 shrinkage radiate in cracks from the centre; these cracks 

 become filled in with crystallised carbonate of lime. Frequently 

 a thin section of a septaria is polished and mounted as an orna- 

 mental table. In these clays are also deposits of jet, which is 

 a lustrous variety of coal, being fossilised wood which has 



