GEOLOGY. 305 



large crinoiclal column, which had been split open by the 

 growth of the sponge. Hunt has called attention to the 

 fact that the abundant abates in the argillaceous beds of the 

 Keweenian or copper-bearing series of Lake Superior, on Michi- 

 picoten Island, are chiefly of two or three symmetrical forms, 

 which he has referred to fossil sponges. They sometimes form 

 geodes, or are wholly or in part filled with calcite or green- 

 earth instead of agate. The same series of rocks exhibits 

 slender cylindrical shapes of agate, which resemble Scolithus, 

 but are often bifurcated, and are conjectured to be of organic 



origin. 



RECENT FORMATION OF MINERALS. 



Daubree has continued and summed up his studies on the 

 mineral species which have been formed in the masonry con- 

 structed by the ancient Romans around various thermal 

 springs, at Plombie.res, Luxeuil, and Bourbonne, in France, 

 and at Oran, in Algeria. The masonry in all of these con- 

 sisted of a concrete of fragments of sandstone, limestone, 

 and brick, in a cement of lime -mortar. In the interstices 

 of the cement have been found crystallized apophyllite, gis- 

 mondine, and scolezite, with opal or hyalite; and also a trans- 

 parent gelatinous matter, which apparently changed in drying 

 into mammillated fibrous masses of a hydrous silicate of lime, 

 having the composition of okenite, mixed, however, with opal. 

 Crystallized arragonite, calcite, and fluor-spar are also found, 

 and a hydrous amorphous silicate of alumina, halloysite. 



The bricks in the concrete are found infiltrated to a m-eat- 

 er or less depth, the cavities occurring in the unaltered por- 

 tions having become more or less filled with mineral species, 

 in part different from those found in the cement. Among 

 these are well-defined crystals of chabazite, mesotype, opal, 

 chalcedonic quartz, and, probably, phillipsite and tridymite, 

 all of which have been formed at temperatures not above 

 70 C. The changes effected in the bricks have been investi- 

 gated both chemically and microscopically. In the infiltrat- 

 ed bricks the proportion of combined water w r as augmented 

 from two or three to eight per cent., and the amount of sol- 

 uble zeolitic matter was found equal to fourteen or fifteen 

 per cent, of the mass. Daubree concludes that the lime, 

 alumina, and silica of the mineral species have been de- 

 rived from the mortar and the bricks, the mineral waters 



