9,S'^ 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



16. Rhombenporphyry xenolith, from granite, northern part of Pine 

 Hill, West Quincy, Mas.s. Analyst, C. H. Warren. 



15. Rhombenporphyry, — Pine Hill, Mass. Analyst C. H. Warren. 



17. Fine-granite type of xenolith, most quartoze type, Hardwick Quarry, 

 Quincy, Mass. Analyst, C. H. Warren. 



3. Granite from Hardwick Quarry. Analyst, H. S. Washington. 



from the greater amount of pyroxene and hornblende and the presence 

 of the Na^Fea" Si40i2 molecule; AI2O3 is lower. Soda is here con- 

 siderably in excess of the potash for the same reason. Lime is lower, 

 though still relatively high for the Quincy rocks owing to the presence 

 of the lime and ferrous-iron rich, augite. If we attempt to calculate 

 the mineral composition for the groups, — feldspars, pyroxenes + 

 hornblende, and accessories, — on the assumption that the proportions 

 of potash feldspar to Ab + An is the same as in the rhombenporphyry 

 (No. 15) it becomes at once apparent that much too great an amount 



