F. Ward — Lighthouse Granite near New Haven, Conn. 137 



in small amounts with no unusual characters ; the quantity of 

 the latter is small even for an accessory mineral. 



Chemical Composition. — The chemical composition of this 

 granite is given below in column I: the other analyses are 

 inserted for comparison : 



I II III IV V 



Si0 o 72-47 72-47 73-05 71*23 73'93 



A1„0 3 14-73 14-78 14-53 13*64 12 29 



Fe~0 3 -96 -57 2-96 1*70 2-91 



FeO._ -97 2-00 1*00 1*55 



MgO Tr. *34 Tr. -75 -04 



CaO *81 1*27 2*06 2-31 *31 



K 2 5*61 4*53 5*39 3*79 4*63 



Na a O 3*69 4*03 1*72 3'55 4*66 



P Q U 04 Tr. — — — 



H 2 0° *72 -48 -29 1-72 -41 



C0 2 .. -10 Tr. — -05 — 



TiO„-_. _ n.d. n.d. — -21 -18 



100-10 100-4' 



100-00 



99-95 100-91 



I. — Lighthouse Granite, aual. F. Ward. 

 II. — Branford Granite, anal. F. Ward. 



III. — Westerly (red) Granite, anal. F. W. Love ; used by J. F. Kemp, 

 Bull. Geol. Soc. Amer., x, 375. 



IV.— Conanicut Granite, anal. L. V. Pirsson, this Journal, xlvi, 373, 1893. 

 V. — Quincy Granite, anal. H. S. Washington, this Journal, vi, 181, 1898. 



The rock belongs decidedly to the alkalic group of granites, 

 as is shown by the small amount of R" elements, particularly 

 lime, and the high alkali content. It is noticeable that the 

 potash is greatly in excess of the soda. 



Alteration. — This, while appearing in all specimens to some 

 degree, is not extensive : none of the ledges is so far 

 weathered, for instance, as to show any residual soil formation. 



In the hand specimen the alteration is shown by the change 

 of the feldspars. They lose their luster and become whiter 

 (are kaolinized), and the pink color of the orthoclase is apt to 

 be lost. The development of hydrated iron oxide also stains 

 the rock a brownish color. However, quarry specimens taken 

 only a few feet down from the surface will not show these 

 signs of weathering ; in many cases specimens only a foot from 

 the surface will appear fresh to the eye. But along joint or 

 fracture planes the weathering has been more active and there 

 the rock may be enough decayed to crumble under slight 

 pressure. 



Under the microscope the feldspars which appeared fresh 

 to the eye are found to be clouded to some extent with kaolin : 

 further evidence of change is seen in the development of seri- 



Am. Jour. Sci.— Fourth Series, Vol. XXVIII, No. 164.— August, 1909. 

 10 



