REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 127 

 ARCHEAN ROCKS. 



No. 11. Hornblende-Granitic-Gneiss. 



This rock-mass which has its greatest developnient in 

 Middleton, Boxford and Georgetown, has the appearance 

 of an ancient rock by being greatly folded and crumpled, 

 by being cut by veins and tongues of diorite and granite 

 rocks and, also, from its position, being in part below the 

 lower cambrian gneisses. Conclusions made from these 

 field evidences indicate that it is one of the oldest rock- 

 masses of the region, and it should be placed in archean 

 time, the equivalent of the Canadian Laurentian period. 



No. 12. Porphyritic-Granitic-Gneiss. 



This formation occurs in Georgetown, West Newbury 

 and Amesbury. It is much like the last one described, 

 but contains numerous large porphyritic crystals of mi- 

 crocline which are invariably developed across the plane 

 of the stratification of the rock-mass. The whole area 

 has been subjected to great strain by a down throw fault 

 in the river Parker Valley between Georgetown and West 

 Newbury. This strain is seen in the large porphyritic 

 crystals, nearly all of them being cracked, bent or broken. 

 For fuller account of these last two gneisses (Nos. 11 and 

 12) see Bulletin of the Essex Institute, Yol. xxn, Geo- 

 logical and Mineralogical Notes No. 2. 



ARKOSE : CONGLOMERATE-GRANITE. 



No. 13. Muscovite-Granitic-Gneiss. 



During the past season many additional thin sections 

 of the rocks from all over the area where this formation 

 occurs have been studied in the laboratory with the mi- 

 croscope, throwing much light upon this otherwise little 



