CHEMICAL WOilK. 



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Thinolite : from Lake Mono. I. C. Russell. 



Some of the travertine deposits of Gardiners River and elsewhere are a 

 result of the growth and secretions of Conferva-like plants, as explained bv 

 W. H. Weed. 



In the Lahontan and 138. 



Mono basins, as described 

 by King and later by 

 Russell, he material has 

 often a crystalline form, 

 the origin of which is 

 yet unexplained : this 

 variety is the thinolite 

 of King. A common 

 form is represented in 

 Pig. 138. 



The beautiful trans- 

 lucent limestone of Te- 

 cali, Mexico, often 

 wrongly called onyx, because banded in colors when polished, is a calcareous 

 deposit failing of the coarse and irregular grain of travertine. 



(6) Consolidation. — Of still greater geological range is the cementing 

 work done by calcareous waters. Ordinary sea water, especially where 

 shells and corals abound, consolidates sands made from coral and shell into 

 limestone. The beach sands, drifted sands, and sands over the reefs, when 

 drying from exposure to the air, become cemented in this way. Conglom- 

 erates are also made of broken corals, shells, and calcareous or other pebbles, 

 and breccias, in this, as in other ages, out of a talus or any accumulation of 

 limestone blocks. 



The under-water calcareous sands, as those about coral reefs, also become 

 cemented by the same means, but into a compact limestone like ordinary 

 limestones, showing usually no sand-like grains in the texture. 



(c) Dolomyte-mciking. — Even dolomyte, (sCaiMg) OgC, owes its origin at 

 times — if not always — to the conditions that exist in the history of coral 

 reefs when the magnesia, required to make the calcareous grains magnesian, 

 could have had no source but the ocean. One case of the kind is reported 

 by the author (1849) from the island of Metia, an elevated atoll north of 

 Tahiti {Corals and Coral Islands, page 393). The rock is a compact white 

 limestone. An analysis by B. Silliman proved that it contained 38*07 per 

 cent of magnesium carbonate, the rest being calcium carbonate. The very 

 fine texture of the rock indicates that it was made of the finest of calcareous 

 ooze or mud, such as forms through gentle wave-action in shalloAV lagoons ^ 

 and in such lagoons, mainly shut off from the sea, and therefore in a 

 "salt-pan" condition (page 120), the concentrated brines contained the 

 magnesium chloride and sulphate in a state that favored the formation of 

 dolomyte. 



