105 



HORNBLENDIC GNEISS, ETC., STRATIFIED GROUP. 1 



The petrosilex in Saugus, along the road from Lynn to North 

 Saugus, becomes more quartzose northward, approaching 

 quartzite. This petrosilex, as already observed, shows traces 

 of banding, and the quartzite is distinctly stratified. Occa- 

 sional beds of the quartzite are slightly micaceous, and these 

 become interstratified with large masses of fine-grained horn- 

 blendic rocks, — diorite and hornblendic gneiss, the former 

 predominating. These various rocks appear to be conforma- 

 ble, and the dip is steep to the north-west. The stratification, 

 however, is much disturbed, and in considerable portions of the 

 diorite it is wholly wanting to the eye, and these are then in- 

 distinguishable from the eruptive diorites just described. 



This belt of stratified rocks is cut off toward the west by 

 eruptive granite, but reappears beyond the Newburyport turn- 

 pike, and extends along the north side of Central Brook into 

 Melrose. On Main Street, near the brook, there are large beds 

 of massive quartzite interstratified with felsitic and micaceous 

 beds, and passing on the north into dioritic rocks as before. 

 The strike and dip remain unchanged. The contact of this 

 series with the granite on the high hills half a mile east of Main 

 Street, affords, as previously stated, conclusive evidence of 

 the extravasation of the latter rock ; in this direction also the 

 bedding is much disturbed, and in the hornblendic rocks espe- 

 cially it becomes more and more obscure, and is finally lost ; 

 another example connecting the stratified with the unstratified 

 diorite. The quartzose portion of the diorite — the horn- 

 blendic gneiss — is converted pari passu into fine-grained 

 hornblendic granite ; and here we find a complete explanation 

 of the intricate and intimate relations of the granite and diorite 

 in the so-called " mixed" series. For in the first stage of the 

 mixing the rocks are interstratified, with frequent alternations of 

 the strata and perfect transitions between them ; and partial 



1 The stratified group is represented on the map by color laid on in lines, and these 

 lines have in every case the direction of the average strike. 



