IGNEOUS ROCKS 



469 



The contact with the Milford biotite granite is well exposed at various 

 points south of Copper Mine Hill and is a chilled contact. For OA^er an 

 inch from the contact the rock is very dark in color and perfectly dense ; 

 then quartz phenocrysts appear, but the rock does not assume its cus- 

 tomary texture for about 5 inches. In the porphyry are many stringers 

 of pre-Cambrian schist. In one of these inclusions is found a part of a 

 labradorite porphyry dike. 



An analysis of the porphyry from this neighborhood has been made by 

 J. H. Perry and has been quoted and commented on above under the 

 description of the granite. 



The granite porphyry on Hunting Hill and on the hill to the west are 

 essentially alike that just described. The rocks are badly altered and 



N,0/| Grants Mills granite |0QD| Vein quartz 



loool Felsite 



^li^l Quartz diorite 



Cumberlandite 



Gabbro 



Ashton schist 



Narragansett series 



Riebeckite granite 



Milford granite 



Figure 2. — Oeological Section through Copt)er Mine HhU 

 See A A on map, page 437 



consequently show now no riebeckite nor gegirite, but in their place is 

 found abundant magnetite, together with other alteration poducts. The 

 phenocrysts of microperthite are fairly abundant, the quartz being usually 

 larger than the feldspar. 



From the mineral and textural character described and from their 

 field relations, it seems clear to the writers that the riebeckite-segirite 

 rocks represent the upward projections of a single intrusion of a siliceous 

 alkaline magma, and that the porphyritic phases represent simply the 

 more quickly cooled marginal facies of the magma. Around the margin 

 of the granite, as on the southeast, the rock becomes a porphyry. The 

 granite and porphyry cut only the pre-Cambrian metamorphics and the 

 granite of the Milford type. The stock does not appear to have been 



