177 



GENERAL RELATIONS OF THE SHAWMUT GROUP. 



Dr. T. Sterry Hunt, failing, it seems to me, to appreciate the 

 distinctive characters, mineralogical and petrological, of the 

 amygdaloid division of the Shawmut group, has referred this 

 important set of beds, apparently in toto, to the Huronian series. 

 In 1870^ he divided the rocks in the vicinity of Boston into three 

 classes : ' ' A, the crystalline stratified rocks ; B, the eruptive gran- 

 ites ; C, the unaltered slates, sandstones, and conglomerates." 

 In the class A he recognized two divisions, the first of which 

 comprises the acidic rocks, petrosilex, eurite, etc. ; while the 

 second " includes a series of dioritic and chloritic rocks, gener- 

 ally greenish in color, sometimes schistose, and frequently 

 amygdaloidal. They often contain epiclote, quartz, and cal- 

 cite, and occasionally actinolite, amianthus, scaly chlorite, and 

 copper pyrites. This series holds a bed of dolomite at Stone- 

 ham, and serpentine in Lynnfield, where bedded serpentines, 

 dipping at a high angle to the N.W., occur apparently in the 

 strike of these dioritic and epidotic rocks, which include the 

 greenstones of Dr. Hitchcock, described by him as occasionally 

 schistose, and passing into hornblende slate ; and also his 

 varioloid wacke, under which name he describes the green and 

 chocolate-colored amygdaloidal epidotic and chloritic rocks of 

 Brighton." Having thus clearly grouped together as belonging 

 to one and the same series the amygdaloids, and the diorites 

 and dolomites here referred to the Huronian, Dr. Hunt pro- 

 ceeded, in the following year, as noticed ante, p. 25, to 

 include the whole, together with the acidic rocks named 

 above, in the great Huronian system. 



The basic Huronian rocks of Eastern Massachusetts are 

 rarely chloritic, and very rarely conspicuously so ; in this 

 region this mineralogical character is found well developed 

 in the amygdaloids alone ; and the correlation of these imper- 

 fectly crystalline sediments with the crystalline Huronian dio- 



iProc. Bost. Soc. Nat. Hist., xiv., 45-46. 



OCCAS. PAPERS B. S. N. H. — III. 12 



