CHANGES THROUGH CONTACT-METAMOKPHISM. 279 



In a i)it on tlio Aslnnore farm tlie best exposure oecurs, sliowin*;- both 

 the limestone and gabbro. The hitter roek lias the same character as 

 the marginal portions seen elsewhere, being comi)arativel3' fine, light 

 colored anil showing many l)lack S])ots. Between the gabbro and the 

 limestone tliere is, as at J^onaparte lake, an intermediate zone of contact- 

 products, usually from one to two feet wide. This varies in precise detail 

 in various parts of the i)it, but where best shown there is, next to and 

 grading into the gabbro, a layer containing much woUastonite. Next to 

 this, on the other side, is a mixture of feldspar, pyroxene, scapolite,sphene, 

 zircon, etcetera. Then comes coarse calcite with much pyroxene, and, 

 finally, fairly pure, coarse calcite, which shades off into the ordinary 

 limestone. The different layers are not at all distinct, but shade into 

 each other and vary greatly in relative and absolute thickness. At an- 

 other point the portion of the contact-zone next to the gabbro consists of 

 almost pure scapolite instead of woUastonite. Irregular seams in the 

 gabbro are abundant, and are lined with pyroxene, orthoclase and scapo- 

 lite. Similar facts are presented in the other pits of the vicinity, but 

 owing to the causes above named they are less clearly shown. 



The position of this zone, following all the irregularities of the contact 

 l^etween the two formations, is sufficient proof that it has been formed 

 by the action of the one upon the other. This action can be explained 

 in no other way than as a case of contact-metamorphism resulting from 

 the intrusion of the gabbro into the limestone. Tliis conclusion is en- 

 tirely supported by the mineralogic composition of the contact-zone, as 

 the species named are all recognized contact-minerals, particularly in 

 limestones. 



Xo extended study of the minerals of the contact has been made, and 

 their i)resence in most mineral collections renders unnecessary a detailed 

 descrijition, l)ut it may be well to state some of their more important 

 features. 



Orthoclase occurs in implanted crystals and irregular masses in the 

 limestone. The crystals are opacjue white and range from very small 

 size up to two inches in greatest diameter. They are tabular, parallel to 

 the clinopinacijid, and measurements made with the hand goniometer 

 show the presence of the following faces (^using Dana's lettering) : />, c, ??i, 

 n, 0, 2, y. 



The determination as orthoclase rather than microcline is based not 

 upon measurement, but upon extinction angles and the absence of 

 twinning in cleavage plates. Such examination does, however, show the 

 presence of the microperthitic intergrowth. It also shows in the ortho- 

 tlase from one opening great (juantities of tluid inclusions. These inclu- 

 sions are arranged in rows, and the cavities are so close together that 



XXXIX-Uii.i.. fiEoi.. Hor. Am.. Vol. fi. 1M1»4. 



