GEOLOGY OF FRANKLIN COUNTY 11T 



elusions of small size caught up by the anorthosite from the in- 

 closing rocks. Others may either be dikes or somewhat larger 

 inclusions. In two cases the resemblance to dikes is strong, but 

 the exposures are not sufficiently extensive to furnish a demon- 

 stration, so that the matter must be left open for the present. 



In addition to the above granites there are sometimes found, 

 cutting the anorthosites, curious dikes of a fairly acid rock with 

 porphyritic feldspars and finely granular ground mass which re- 

 sembles somewhat some of the later syenite porphyry. The 

 ground mass is evenly granular and appears to have been com- 

 pletely recrystallized. These dikes have not been thoroughly 

 studied as yet, and are at present regarded somewhat doubtfully 

 as syenite dikes, whose slight width is accountable for their por- 

 phyritic character. 



Diabase. Much later than all the foregoing are rocks which 

 make comparatively small showing in Franklin county. While 

 of several subordinate types, they may all be conveniently classed 

 as diabases, the distinction between them and the hyperites not 

 being a sharp one. They are said to be much later, since they are 

 not in the least metamorphosed, and have apparently solidified at 

 much less depth than any of the other eruptives; hence a long^ 

 intervening period of erosion is argued. 



These are black, very fine grained, flinty rocks with conchoidal 

 fracture and are very easy of recognition. They occur only in 

 dikes which range from a few inches up to 30 or 40 feet in width, 

 more commonly being from 1 to 10 feet, are nearly vertical, and 

 have an east and west trend, small offshoots only excepted. Near 

 the wall rocks cooling has been so rapid as to forbid much crystal- 

 lization, and the rock is glassy. The considerable variation in 

 grain from the sides to the center of these dikes, showing in all 

 but the exceedingly narrow ones, is one of their most distinctive 

 characters, and is in marked contrast to the even grain of the 

 hyperite dikes. They weather out into small, sharp-edged blocks 

 along planes produced by contraction on cooling. They are not 

 likely to be confused with any of the other rocks except possibly 

 some of the hyperite dikes, but these last, in addition to their 



