November 10, 1916] 



SCIENCE 



689 



tion produced by ground-water is only local 

 and very imperfect. Some of the features 

 which lend weight to this view are as follows: 



(1) Recent dolomitized coral reefs are known 

 to have been formed by the reaction of the 

 magnesia of sea-water with the limestone. 



(2) The dolomite areas of mottled limestones 

 are believed to have undergone recrystalliza- 

 tion at the same time as the associated lime- 

 stone areas, as suggested by the occasional de- 

 velopment of zonal growths of calcite and dol- 

 omite. (3) In imperfectly altered limestones 

 the dolomite is seen to follow original lines of 

 weakness rather than secondary structures, 

 such as joints or fractures. (4) In most cases 

 of mottling the dolomitization appears to have 

 progressed uniformly as we should expect it 

 to do in an unrecrystallized rock, rather than 

 to have progressed by forming veinlets and 

 stringers in the early stages. (5) The exist- 

 ence of perfect rhombs of dolomite in many 

 imperfectly altered limestones suggests that 

 the latter had not yet solidified when the dolo- 

 mite rhombs were formed. (6) The wide- 

 spread extent and nearly uniform composition 

 of many dolomites indicates that they must 

 have been formed by an agent capable of 

 operating uniformly over wide areas. (7) An 

 adequate source of magnesia for transforming 

 extensive limestone formations into dolomite 

 is found only in the sea, which contains many 

 times as much of the constituent as ordinary 

 ground-water. (8) Many dolomites are di- 

 rectly and regularly overlain by pure lime- 

 stone formations or by thick shale beds, prov- 

 ing that they must have been formed before 

 these overlying beds were deposited and that 

 descending ground-water has not been influen- 

 tial in their production. 



The evidence of dolomitization beneath the 

 sea then must be considered as positive, but 

 the controlling factors of the process are very 

 imperfectly understood, due chiefly to the lack 

 of careful study of the phenomenon in the 

 modern seas. A thorough investigation of the 

 conditions which favor the transformation in 

 the sea to-day would be invaluable in inter- 

 preting the history of the ancient dolomites. 

 It is believed that very important data bear- 



ing on the problem could be obtained from a 

 more careful study of the coral islands of the 

 Southern Pacific. 



As to whether dolomitization takes place in 

 concentrated seas or not there has been con- 

 siderable disagreement. Until recently the 

 tendency has been to follow Dana, who be- 

 lieved that dolomitized portions of recent coral 

 reefs were formed in concentrated lagoons and 

 assumed that the ancient dolomites must have 

 been formed under similar conditions, but 

 Skeats pointed out in 1905 2 that the outer 

 parts of certain fringing reefs of the South 

 Sea Islands, which face the open ocean, are 

 occasionally dolomitized and that the dolomi- 

 tization of coral reefs is not confined to the 

 lagoons; and Philippi 3 soon after presented 

 evidence of recent dolomitization in the open 

 sea. Still more recently, Blackwelder 4 has 

 given it as his opinion that the Bighorn dolo- 

 mite has resulted from the progressive altera- 

 tion of limestone during deposition, the con- 

 centration of the magnesia being not more 

 than two or three times as great as in the pres- 

 ent ocean, since more than this amount would 

 have been unfavorable to the life processes of 

 the time. There are many commendable 

 points to this theory of progressive dolomitiza- 

 tion at low concentrations, but if dolomitiza- 

 tion can go on under these conditions, why 

 are not all of our limestones dolomitic? In 

 answer to this query it might be said that the 

 alteration takes place under unusual circum- 

 stances, possibly through the agency of cer- 

 tain bacteria which are not always present 

 when limestone is deposited. 5 But much of 

 the field evidence speaks against progressive 

 dolomitization. The wavy boundaries some- 

 times exhibited between the dolomitic and 

 non-dolomitic portions of formations; the 

 lateral gradation of beds of dolomite into 

 limestone; pseudo-interstratification effects of 



2 Quart. Jour. Geol. Soc. London, Vol. LXI., p. 

 97, 1905. 



iNeues Jahro., Festband, Vol. I., 1907, p. 397. 



*Bull. Geol. Soc. America, Vol. XXIV., p. 607, 

 1913. 



« Both Nadeon and Walther have suggested the 

 possible influence of bacteria in dolomitization. 

 See "Gesichte der Erde und des Lebens," p. 90. 



