INTIJOD'CTIOX 4^35 



lack of FeO in sedimeutarv culciic? The additional evidence of analyses 

 23resented later suggest tbat tliese conditions are the rale. 



PREPARATION OF LEMBERG STAIN 



The stanctard Lemberg solution is prepared by boiling for 20 minutes 

 a mixture of 4 grams AlCl.., G grams extract ol^ logwood, and GO grams 

 of water, with constant stirring aiid addition ol' water to make up for loss 

 by evaporation. Tt is tbeu filtei-ed and bottled for use. Calcite, when 

 immersed in it, Ijecomes coated with Al(OH)., which absorbs the dye and 

 acts as a mordant. Dolomite is coated iu aljout 30 minutes. When pre- 

 pared in this way Avith fresh AKT. crystals, it coated calcite almost in- 

 stantly with a thick blue gum blistered with carbon dioxide bubbles. 

 When washed the gum peeled off, and on drying it cracked. The standard 

 preparation diluted A\'ith 1,'3U0 cc. of water gave perfect results. 



. TlMl'] or DOLOMJTIZATIOX 

 TWO UYPOTHEHES 



Two hypotheses regarding tlio time at wJiicli a dolojuite developed can 

 be formulated. («) Tt may have developed in the sea as a sediuient either 

 by direct precipitation or by reactions within the limy sediruents of the 

 sea-bottom, {b) It may have been formed l)y tlic; replacement of lime- 

 stones through the action of \Aaters underground after the limestones had 

 emerged from the sea. It is also conceivable tJiat tlie dolomitization of 

 a formation began in the sea and was carried farther in post-emergent 

 times. Evidence as to the time of dolomitization in specific cases is some- 

 times difficult to obtain. 



POST-EMERGENT DOLOMITES 



Proof that some dolomite masses are post-emergent in their time of 

 development lies in the fact tluit tliey J'ej)lace limestones adjacent to faults 

 and fissures, ^ye\\ Iviiowj) cases of tliis kind are found in the Joplin zinc 

 district and at Aspen, C'olorado. Dolomite vein materials also belong to 

 this class. Small deposits of the latter are found in almost any dolomite 

 quarry of the Central States. Altliongh common, the dolomites developed 

 under emergent conditions seem to include only a minor portion of dolo- 

 mite dejjosits. 



PRE-EMERGENT DOLOMITES 



Evidence from stains and oilier inrlliods of direct study. — That the ma- 

 jority of dolomites were formed as marine sediments is shoAvn by the fol- 

 lowing facts : 



XXXI — Bull. Geol. Soc. A>r., Vol, 28, lOlG 



