q6 Calcareous Geüus. The refiduurii, after folution, being well waffieä and dried, weighs froin £ to | of the whole. It confifts of mild calx and clay in the fame proportion as the former variety, and frequently a large proportion of marine fliells. Moll rriarls contain a fruail proportion of magnefia, and fome veftiges of marine acid *; they are liable alfo to various other contaminations. By expofure to the air and moifture, it foonef or later, generally at fartheft within a yeär, chips and falls to pieces. This difintegration is re- markable, for it does not proceed folely from the abforption of water, and the dilatation and con- traftion of this abforbed water, by the varying temperature of the atmofphere, as it täkes place in climates under which this Variation is not con- fiderable; nor does it procetd from the abforption of air by ferruginous particles it may contain; for marls that contain fcarcely any iron, or which contain iron in a flate that admits of'no farther calcination, equally fall to pieces after expofure to the atmofphere, and fo do the fliells contained in it. I fufpeft it to proceed partly fiom the ab- forption of moiflurej and partly from the abforp tion of air by the carbonic principle of the ma rine exuvise, or fliells found in the märl. Marls that contain only 66 per ct. of mild calx are frequently burned to obtain lime; this lime takes lefs fand indeed, but is unfit for con- ftruäions under water. D. Wettering remarked, that marls, which contain above 4. of their weight of aerated calx, will not form bricks. Dr. Hig- gins obferved that mortar, which contained -J- of of its weight of argill, was lubjed to chip by * Per Monnct, 4 Roz. 180. expofure