532 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



which we consider entirely decisive of the question, and in favor of the 

 mineral nature of the Eozoon. The fact of the occurrence of the so- 

 called "eozodnal structure" in some of the limestone deposits of East- 

 ern Massachusetts has been repeatedly admitted both by the advocates 

 and the opponents of the organic character of this peculiar body.* By 

 most geologists these calcareous masses have been considered to be in- 

 terbedded with and an original portion of the formation, and supposed 

 to be beyond doubt of sedimentary origin. 



In 1871, however, after a careful study of the eozooual limestones of 

 Chelmsford, Bolton, and Boxboi'ough, Mr. L. S. Burbank claimed that 

 these were not true stratified deposits laid down with the gneiss, but of 

 subsequent formation to that rock, and of vein-like character ; and that, 

 in consequence of this, the Eozoon must be of mineral, and not of ani- 

 mal origin. These views were sustained by Messrs. W. H. Nilcs and 

 J. B. Perry, — the latter extending his observations so as to include the 

 locality at Newbury, Mr. Burbank gave figures illustrating the mode 

 of occurrence of the limestone, and these seemed to furnish strong evi- 

 dence in support of his views.f 



Most of these localities have been studied by us, with especial refer- 

 ence to the origin and mode of occurrence of the calcareous material. 

 At Stoneham, the limestone was found to occur in an indurated argil- 

 lite, which is cut through by dikes of diabase (diorito), etc. The cal- 

 careous masSj lying approximately in the direction of the strike of the 

 enclosing rock, frequently cuts across its lamination planes, so as to 

 include masses of the argillite; and portions sometimes extend for a 

 considerable distance across the stratification, so as to have the charac- 

 ter of fissure veins. The limestone is entirely crystalline, and shows 

 upon its borders that intense chemical action has there taken place, as 

 it is greatly altered and filled with epidote and such other silicates 

 as are ordinarily found resulting from conditions of the kind here sug- 

 gested. While one of the dikes was seen to intersect the limestone, 

 others were found to have been cut off by it. These latter dikes 

 abutted directly against the limestone, and along the line of junction of 

 the two rocks were the same evidences of intense chemical action which 

 were seen at the edge of the calcareous mass in other places. 



From this it follows that this limestone is a segregated or vein-like 

 deposit, of earlier date than some of the dikes, but posterior to the 



* See anUf pp. 411, 419. 



t Proc. Bost. Soc. Nat. Hist., 1871, XIV., 189-204; Proc. Amer. Assoc. Adv. 



Sci., 1871, XX., 262-2GG, 270-276. 



