328 



THE POPULAR EDUCATOR. 



LESSONS IN GEOLOGY. XXI. 



THE TKIASSIC GROUP (continued). 



THE plants of the triassic period bear a strong analogy to the 

 fauna of the lias and oolite above, chiefly consisting of ferns, 

 cycads, and conifers, and in some localities at least the triassic 

 forests must have been comparable to those of the carboniferous 

 times ; for near Richmond, in Virginia, there is a coal-field, some 

 twenty-six miles long, belonging to the trias. The coal it yields 

 is equal to that of our Newcastle coal-field. The bed is very 

 thick, some of the chambers in the mine being 40 feet high. 

 The Equisetum columnare, which is common in our English 

 trias and oolite, is of very frequent occurrence in the Rich- 

 mond beds, and is often in an upright position. The sketch we 

 furnish of a triassic forest will give an idea of the flora of the 

 period. 



Although the trias yields with us no coal, yet it is valuable 



then only to call in the ordinary alteration of level to allow 

 a deposit of sedimentary matter to cover the whole, and we 

 should have all the necessary conditions for a salt-mine. 



But a difficulty at once meets us. It is evident that the 

 saline deposit at the bottom of the evaporated lagoon would 

 not be pure salt, but it would be a mixture of all the salts 

 contained in sea-water, whereas the rock-salt we find in tho 

 strata is pure salt (chloride of sodium). And, moreover, in the 

 evaporating lagoon, when the water reached & certain specific 

 gravity, not being able to contain its salts in solution, they 

 would crystallise out ; but the sulphates of lime and magnesia 

 and the chloride of magnesium would be deposited first, and the 

 salt (the chloride of sodium) the last. If, therefore, the de- 

 posits of salt owe their origin to this process, we should expect 

 to find, immediately beneath the salt, layers of gypsum, etc., but 

 this is not the case. Moreover, the salt appears not so much in 

 layers as in masses, and if any gypsum be in its neighbourhood 



IDEAL FLORA AND FAUNA OF THE TRIASSIC PERIOD. 

 1, Albertia; 2, Cycadese; S, Voltzia; 4, Ferns. The animal in the foreground is the Microlestes autiquus. 



for its deposits of salt. All our salt is procured from the salt- 

 mines of Cheshire and Worcester. The chief deposits lie in 

 the valley of the Weaver, near Northwich. Here the salt 

 reaches the great thickness of 170 feet. A layer of indurated 

 clay separates the salt into two beds, but is itself traversed by 

 veins of salt. 



Just as the coal is not peculiar to the carboniferous, neither 

 does the trias exclusively contain the deposits of salt. The 

 mineral is found in the oolite in the Salzburg Alps, in the 

 greensands at Cordova, and the celebrated Polish mines of 

 Wielicza are in the tertiary ; and briny springs from the car- 

 boniferous and Devonian give evidence that the oldest of the 

 formations also contain it. The origin of the deposits 6f this 

 useful mineral is a question surrounded with difficulty. We 

 know that if a sheet of salt water were continually exposed to 

 the sun, evaporation of the fresh water would continue until 

 all the saline ingredients were left, and the bed of the lake 

 would become dry. If this lake were a lagoon separated from 

 the sea by a sand-bar, so high that only very rarely a high tide 

 or a storm could replenish its exhausted water with more salt 

 water, the evaporation might go on for centuries, and an in- 

 ibSfcite ruar.tlty o^ saline matter would accumulate. We ha.ve 



it is generally found alove, and not below. Yet, although this 

 crystallisation theory may not of itself be sufficient to answer 

 the question, it evidently played no small part in the deposition 

 of salt, for the ripple-marks and footprints on the sandstones 

 occur at all levels around the salt, showing that shores and 

 shallow water were in the vicinity as the deposition progressed. 



Subterranean heat has been called in to help the elucidation 

 of the difficulty, and metamorphic action has been relied on 

 to make the consolidated salt assume a rude crystallisation an<i 

 a homogeneous character, but as yet observation does not fully 

 agree with the supposed results of any theory. 



Professor Jukes gives the following list of generic forma 

 making their first appearance in the triassic period the dawiv 

 of the mesozoic age : 



Plants. CEthophyllum, Albertia anomopteris, PteropJiyllum, Dictyo- 



phyllnin. 



Brachiopoda. Koninckia, Tliecidium. 

 Conchifera. Ostrsea, *Gervilia Myophoria, *Isoarca, Opis, Trigonia, 



Myoconchus plicatula. 

 Gasteropoda. *Scoliostoii>a, Naticella, N-jrinaja, *Platystoma. 



* Those marked * did not outlive the period. 



