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



known class of rocks. The conclusion reached is that 

 these rocks are arkoses and belong to a series of more or 

 less crushed granite conglomerates which have been 

 washed and reconsolidated from the decay of the nmsco- 

 vite-biotite-granite of the region, or from some similar 

 rock farther to the north. 



SCHISTOSE FOLIATED ROCKS. 



No. 14. Amphibolite-Gneiss. 



Where this rock-mass occurs in the diorite area it is 

 clearly proved to be a flow structure caused by currents 

 of minerals in the diorite magma. This seems to be es- 

 pecially clear in the Peabody and Danvers regions. The 

 outcrop near Crooked pond in Boxford is entirely sur- 

 rounded by the archean gneiss and is probably a remnant 

 of some metamorphosed Cambrian slate. This is also 

 probably the case at Rooty piain in Rowley. The outcrop 

 in Newbury of this rock-mass fromits position, interstrat- 

 ified with limestones, slates and gneisses, is of undoubted 

 Lower Cambrian age, a metamorphosed Cambrian slate. 



CRYSTALLINA ROCKS STRATIFIED. METAMORPHIC ROCKS 

 OF CLASTIC ORIGIN. 



No. 15. Mica-Schist and Sandstone. 



These two rock-masses are invariably interstratified and 

 in some places are seen as members of the Lower Cam- 

 brian Sediments. The schist is undoubtedly a metamor- 

 phosed slate. 



No. 16. Corderite-Gneiss. 



This rock f ormation is another member of the Cambrian 

 series and is well developed in North Andover and north 

 of Bald Pate hill in Georgetown. 



