FIRST VARIETY OF GABBRO. 2G9 



similar. On the other sides tlie gabbro area falls off more gradually. 

 The section at the cliff sliows at the base limestone dii)ping under the 

 next higher member, laminated gneiss. Above this again there is lime- 

 stone, largely cut out by gabbro, which also intersects the gneiss (see 

 figure 3.) Tlie structural relations are perfectly clear and admit no 

 doubt of the intrusive nature of the gabl)ro, which is further shown by a 

 conspicuous amount of contact metamorphism. 



Macroscopic C/iaradcridics. — The most ])ronounced feature of the gabbro 

 as seen in the field is tlie great variation from point to point in composi- 

 tion and structure. On the one hand the constituents may be less than 

 a millimeter in diameter, while on the other they may reach an inch 

 or more in their greatest dimension. Naturally, the finer portions are 

 more conspicuous near the margin, but there are abundant exceptions 

 to this rule. In composition there is a range from a nearly black rock, 

 com})osed almost wholly of ferro-magnesian minerals, to a rock consist- 

 ing chiefly of white feldspar, with a few prismatic pyroxenes. The latter 

 is the least abundant variety, the bulk of the rock being rather dark- 

 colored. It is surprising how many varieties of the gabbro may appear 

 in a small area, the passage between the most extreme phases often tak- 

 ing place within five or six feet. Such variations are, of course, very 

 common in basic igneous rocks, but it is probable that there is nowhere 

 a more striking example of the phenomenon. 



Miner (dog ic. Composition and Characteristics. — The microscope shows the 

 rock to consist of plagioclase feldspar, pale green augite, hornblende, 

 apatite, pyrite and alteration products. All of the minerals, save the 

 apatite, are ailotriomorphic, though in the feldspathic variet}^ the augite 

 has an imperfect prismatic habit. The augite is normally the prevailing 

 ferro-magnesian constituent, though it may be replaced by hornblende. 

 The relation between the two is interesting, for if it is ever safe in the 

 absence of idiomorphic boundaries to say that massive hornljlende is 

 derived from pyroxene, it may be stated in this case. Every stage of 

 the process is clearly shown, beginning with the appearance of small 

 spots of deeper color scattered through the light green of the pyroxene 

 and ending with a complete substitution of hornblende. The change 

 does not begin in any particular portion of the pyroxene, but at numer- 

 ous points scattered through the mass. Professor Iddings * has pointed 

 out the need of caution in accepting such an explanation of the origin of 

 hornblende, but in the present instance the facts are very strong in its 

 sui)port. 



The extinction angles of the feldspar indicate the prevalence of an acid 



» Twelfth Ann. Rep. U, S. Geol. Surv., p. 610. 



