Academy of Sciences] 

 No. n 



AMERICA. 



57 



close of the Ordovician period *'*•;*••■. The latter Appalachian disturbances at the close 

 of the Carboniferous would account for the widespread lamination developed in the peritodites, 

 and would give occasion for the minor later intrusions." This is not, however, offered as a 

 final conclusion on the subject, and possibly the earlier age suggested for the Broughton series 

 of intrusions will stand. A still greater age is claimed for the serpentines more or less associ- 

 ated with the gabbros of the neighborhood of Philadelphia. Miss Bascom ('09) states that these 

 are limited to the region of pre-Cambrian rocks and are probably of pre-Cambrian age, either 



Fig. 17. — The distribution of peridotites, etc., in eastern North America. (After Lewis and Pratt.) 

 1. Schist and gneiss. 2. Peridotites and other magnesian rocks. 3. Paleozoic and later sediments. 



peripheral to the gabbro, or injected into spaces left by contraction of these rocks. These 

 gabbros occur in greater or less amount associated with the serpentines. They are subordinate 

 in the south but in the region about Philadelphia they distinctly predominate over the serpen- 

 tine. They generally have a laccomorphic type of development and are elongated following 

 the strike of the country. A gravity-stratification is sometimes suggested (e. g., Loughlin '12). 

 McCallie ('19) also considers the ultrabasic rocks of Georgia, in the southern part of this zone, 

 are pre-Cambrian. That some of the plutonic complexes of this zone containing ultrabasic and 



