F. Ward — Lighthouse Granite near New Haven, Conn. 133 



J. G. Fercival, in the best map of Connecticut up to 1906* 

 under a description of the Primary rocks, subdivision Ala, 

 separates the Lighthouse-Branford granite from the Stony- 

 Creek granite farther east. He notes the pink feldspar and 

 the gneissic character of the rock in places. While his obser- 

 vations and mapping were remarkably accurate for his time, 

 yet the state of geologic science at that period did not permit 

 any more definite or scientific delineation of the formation. 



J. D. Dana mentionsf a "granite or granite-like gneiss" on 

 Lighthouse Point, but gives no detail of description of the 

 formation or its extent. 



There have been other references to the region by the older 

 geologists, but only in the more or less vague terms " primary", 

 " granite", " gneiss", " metamorphics", crystallines", etc. ; no 

 special descriptions are given and the value of such articles is 

 practically only historical. 



The most complete geological map of Connecticut to-day is 

 that published by the State Survey 4 This, with further 

 description in the Manual of the Geology of Connecticut,§ 

 brings the knowledge of the region up to date. The Light- 

 house granite is here included under the Branford granite- 

 gneiss as one formation. Only a brief outline of the main 

 characters of the mass are given. The authors state that suffi- 

 cient data have not yet been accumulated on which to base a 

 complete description of this rock. 



Field Geology. 



General. — The formation as a whole consists of a medium - 

 grained granite of pink or reddish color. The structure may 

 be gneissic, varying from that well-developed and easily seen 

 in the hand specimen to that which only shows in the mass. 

 The average hand specimen would not show a well-marked 

 gneissic structure. It meets the Triassic sandstones, shales and 

 traps abruptly, the contact throughout being a fault contact. 

 It grades imperceptibly into the neighboring Branford granite. 



The uniform character of the granite of this formation is 

 modified by several features, viz. — pegmatite, aplite, quartz 

 veins, inclusions, as well as some variation along the fault 

 contact. These modifications will be noted in detail, as 

 follows : 



Pegmatite. — Pegmatite is very common throughout the 

 area. In general appearance it may be described briefly as a 



* Report on the Geology of the State of Connecticut, 1842. 



•J- The Four Rocks, with Walks and Drives about New Haven, 1891, p. 89. 



% H. E. Gregory and H. H. Robinson, Preliminary Geological Map of 

 Connecticut, State Geol. Nat. Hist. Surv., Bull. 7. 



§W. N. Rice and H. E. Gregory, State Geol. Nat. Hist. Surv., Bull. 6, 

 pp. 146-147. 



