200 SCIENCE PROGRESS. 



An interesting contribution to thermal metamorphism 

 is given incidentally by Brogger and Backstrom in their 

 memoir on the minerals of the garnet group (16). Under 

 the head of alkali-garnets they place sodalite and its allies, 

 including the natural ultramarine lasurite, the constitution 

 of which is shown to be analogous to that of the garnets 

 proper. The well-known, but hitherto mysterious, lapis 

 lazuli or Lasurstein is found to be a product of thermal 

 metamorphism in a calcareous rock. It consists of lasurite, 

 blue and white haiiyne, diopside, often hornblende and a 

 micaceous mineral, and calcite, with pyrites, etc. 



Some interesting phenomena of metamorphism in cal- 

 careous rocks have been described by J. F. Williams (13) 

 at Magnet Cove in Arkansas, a district already referred to. 

 The crystalline limestone at that locality contains several 

 special minerals referable to thermal metamorphism, the 

 most important being lime-bearing silicates. These include 

 crystals of perofskite or dysanalyte, idocrase, and the lime- 

 magnesia-olivine monticellite, the conditions resembling in 

 some degree those of the classical district of Monzoni in 

 the Tyrol. Other minerals met with are octahedra of mag- 

 netite, masses of apatite-needles, and a green biotite. 



The occurrence of dipyre in the metamorphosed lime- 

 stones of Ariege and the Pyrenees has long been known, 

 and the mode of its occurrence has recently been investi- 

 gated by Lacroix (17). The lherzolites which are the 

 prevalent igneous rocks of the district appear to have been 

 intruded prior to the deposition of the Upper Jurassic strata, 

 which contain pebbles of the lherzolites. The intrusion of 

 these rocks cannot, therefore, have produced the dipyre 

 which is so frequent in the crystalline limestones of the Upper 

 Jurassic. The limestones of the Middle Lias were, however, 

 cut and metamorphosed by the lherzolites. One type of the 

 metamorphosed rocks consists chiefly of large crystal-plates 

 of dipyre enclosing many flakes of biotite and grains of 

 pyroxene, with abundant sphene and more rarely plagio- 

 clase. Another type is very micaceous, irregular crystals 

 of dipyre being disseminated through a mass made up of 

 little flakes of biotite and grains of pyroxene. Rutile, 



