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



very coarse granite. It lias two habits of occurrence — (1) True 

 dikes having definite directions and with well-defined bound- 

 aries. But the contact with the normal granite is never 

 sharp and clear-cut ; at a little distance it may seem to be so, 

 but on closer inspection the pegmatite and granite proper are 

 found to merge one into the other within a distance of half an 

 inch. These dikes vary in width from less than an inch up to 

 as much as ten feet, but six or eight inches is a more common 

 width. They vary in length from ten or fifteen yards to a 

 hundred or more. They may be parallel with the gneiss planes 

 or cut across them in any direction ; occasionally their course 

 is the same as the neighboring joints. In some cases the 

 smaller dikes are lens-like, thinning out within a few yards ; 

 this kind is apt to have its center portion made up of quartz 

 alone, changing gradually to the normal quartz-feldspar 

 mixture as its margin is approached. (2) A large portion of 

 the pegmatite occurs in irregular patches and smears scattered 

 through the granite. They do not extend in definite direc- 

 tions; nor do they have well-defined boundaries, but grade 

 into the surrounding rock. Their size varies from that of one's 

 hand to those several square yards in extent ; or there may 

 be quite large areas which are a heterogeneous mixture of 

 pegmatite and granite. This patchy type of pegmatite can be 

 considered as an intermediate stage between true pegmatite 

 dikes and true miarolitic cavities. The latter, as is well known, 

 are characterized by irregularity of boundary and direction, and 

 only differ from the irregular pegmatite of the area in having 

 cavities in their middle portions. There is no difficulty in 

 distinguishing the patchy pegmatite from the miarolitic type; 

 the relation between them can easily be seen and gradations 

 between the two can well be imagined. No true miarolitic 

 cavities have been found in the granite. 



The two types — definite dikes and irregular, ill-defined 

 patches — suggest two periods of pegmatization ; one occurring 

 while the granitic magma was unconsolidated and in a pasty 

 condition, another occurring when the magma was all but 

 solidified into rock, the latter period of course being the time 

 when the dike type of pegmatite was formed. 



Aplite. — This type of rock is less common than the pegma- 

 tite. Its occurrence is of two kinds — as in the case of the 

 other— dikes and irregular streaks. It has the usual fine-grained, 

 sugary texture. It is simply a finer grained granite with less 

 ferro-magnesian minerals than normal granite. The ferro- 

 magnesian minerals may fail utterly at times. The texture 

 also may be rather coarse occasionally. 



Quartz-veins. — These are common throughout the whole 

 area. Usually they are small, a few feet to a few yards long, 



