CHANGES THROUGH CONTACT-METAMORPHISM. 277 



This resemblance is such that there can be little doubt that the bosses 

 are offshoots from the main mass of gabbro, which owe their modified 

 character to the different conditions under which they have solidified. 

 It should be noted, however, that some of these bosses may be offshoots 

 of the granite which lies to the north. 



Confdd-mctamorphism. — Wliile the structural evidence is in itself suffi- 

 cient to justify the conclusion that the gabbro is intrusive in the lime- 

 stone, there is no lack of substantiation in the way of mineralogic changes- 

 On the contrary, wherever the two formations are observed in actual 

 contact there is seen a considerable amount of contact-metamorphism, 

 both endomorphic and exomorpliic. 



These changes, though varying in minor details from point to point, 

 are on the whole of a rather uniform character, so that some of them may 

 be described as a whole and others may be represented by a few typical 

 localities. 



Endomorphic Clianges. — When the gabbro is examined near the con- 

 tact it is found to be finer grained than the normal rock, though this is a 

 rule to which there are abundant exceptions. At the same time the 

 feldspar often becomes bleaclied to a light gray or white, and there may 

 be an increase in the amount of dark constituents. The result of these 

 changes is a rock finer than the normal gabbro, and, as a whole, lighter 

 colored, but dotted over with numerous black spots. 



Microsections from these portions of the gabbro often have the 

 pyroxene in larger grains than in tlie normal rock, and the grains are in 

 many cases bordered by scales of green hornblende. This hornl)lende 

 looks like a secondary product, but the evidence in support of such a 

 supposition is not as strong as in the case already described. The 

 most striking and characteristic feature of these marginal portions of the 

 rock is the presence of titanite, often in very considerable quantity. This 

 mineral is usually in irregular grains of small size, several of which may 

 be aggregated, forming patches two or three millimeters in diameter. It 

 is noticeable also that sections from this part of the rock show much less 

 granulation of the minerals. 



The plienomena shown in the marginal portions of the main body of 

 gaV)bro are largely repeated in the })osses lying beyond its northern 

 boundary, but they, on account of their small size, have the marginal 

 character tliroughout. This accounts for the variation from tlie normal 

 type shown by the rocks of these bosses, and emphasizes the probability 

 of their being offshoots from the main body. These changes in the 

 character of the gabbro might l)e regarded as an exami)le of the variation 

 from point to i)oint so often shown by members of the gal)bro family and 

 other basic rocks, but this supposition is completely negatived })y tlie 



