TH.\LLOPHYTA 39 



which build the limestone may be destroyed. The studies of 

 J. Walther on a Lithothamnion reef at a depth of one hundred 

 feet in the Bay of Naples have shown that the action of percolat- 

 ing water may obliterate all the structure of the seaweed, leav- 

 ing a structureless limestone. Many examples of such structure- 

 less limestones are known from the Mesozoic and Cenozoic 

 rocks of southern Europe in some of which a few specimens of 

 Lithothamnion are still preserved. Thus it may not always be 

 possible to tell in the study of any given limestone what organ- 

 isms were operative in its upbuilding. In those cases, however, 

 where the species can be recognized, calcareous algae are often 

 valuable as index fossils since there are certain forms with re- 

 stricted geological, but wide geographical, range. Such, for 

 example, is Solenopora co?7t pacta, abundant during the middle 

 Ordovician in Canada (Trenton and Black River formations), 

 in Scotland (Llandeilo-Caradoc), and in northern Europe. It 

 often forms entire beds of limestone ; at times it weathers out 

 as little pealike masses. 



Fossil, banks of calcareous algae are believed to be responsible 

 for certain structures in the Bighorn dolomite of Wyoming. 

 Other fossil reefs on a gigantic scale are now represented, 

 according to many geologists, by the dolomites of the southern 

 Tah-oI. 



Certain algae, the thermal alga^, are responsible for the 

 beautiful siliceous and calcareous deposits of the Yellowstone 

 Park. In some way not wholly understood they cause the depo- 

 sition of the calcium, as at Mammoth Hot Springs, or of the 

 silica, at most of the other hot springs. The beautiful colors 

 of these deposits are hence due to plants, — plants which here 

 live in water of a very high temperature, — between 90° and 

 185° F. The kind and color vary with the temperature, there 

 being representatives of the green algae, the blue-green, etc. In 

 the cooler waters these forms may be recognized as algae, appear- 

 ing in green filaments, or red or brown leathery sheets lining the 

 springs and resembling the seaweeds of the coast. But in the 



