278 SMYTH — ROCKS OV NORTHWESTERN ADIRONDACK REGION. 



fact that the changes always appear as the hmestone is approached and 

 have not been seen in any other part of the rock. 



Exomorphic Changes. — Decidedly more marked than in the gabbro, 

 are the mineralogic changes in the adjacent limestone. A good example 

 is furnished by the contact, previously referred to, in the cliff on Bona- 

 parte lake. The gabbro at this point shows the ordinary marginal char- 

 acter. The limestone at a distance from the contact is distinctly banded, 

 but as it approaches within three or four feet of the gabbro the banding 

 disappears and the limestone becomes a uniform white. Immediately 

 adjoining the gabbro is a zone composed of a fibrous white mineral, with 

 abundant grains of a green mineral. This zone varies in width from an 

 inch up to a foot, and the relative porportions of the different minerals 

 varies in different parts. The minerals sometimes show a banded ar- 

 rangement which is wholly discordant with the original banding of the 

 limestone, but is parallel to the line of contact. This zone is totally 

 different from either of the rocks between which it lies, and is without 

 doubt a product of the action of the heated intrusion upon the lime- 

 stone. The contact-zone seems rather more closely linked to the intru- 

 sive rock than to the limestone ; but it is doubtless to be regarded as 

 an altered portion of the latter, or perhaps more accurately as a product 

 of interaction, deriving some of its constituents from both rocks, and thus 

 in a sense intermediate between the two. Sections from this zone show 

 green pyroxene and wollastonite as prevailing minerals, with smaller 

 quantities of titanite and garnet. 



Similar contacts may be seen at several other places along the shore 

 of Bonaparte lake, and at many points elsewhere in the region. They 

 are particularly marked in the case of the small bosses lying to the north 

 of the main gabbro body. 



In most of these exposures the phenomena are somewhat obscured by 

 weathering, but this difficulty is removed at several points about one 

 mile east of Natural Bridge. The well known minerals from this locality 

 are such as to suggest the possibility of the presence of a contact-zone, 

 and, indeed, it has been stated* that they occur " near the juncture of 

 crystalline and sedimentary rocks." As a matter of fact, the most im- 

 portant openings from which these minerals have been collected were 

 found by the writer to be immediately on the contact of the gabbro and 

 limestone. Some of these pits have been opened to a depth of ten feet 

 or more, with a length of fifteen or twenty feet, and though several have 

 been filled in with bowlders gathered from adjacent fields, two or three 

 are open to inspection and furnish very perfect sections of the contact, 

 with an almost complete absence of weathering. 



* Dana's System of Mineralogy, sixth edition, p. 1063. 



