1891.] NEW YORK ACADEMY OF SCIENCES. 19 



lacking feldspar entirely, and having comparatively little glass. 

 Olivine is occasionally seen in a fresh condition, bnt in most dikes 

 an alteration product not always satisfactory alone indicates its 

 probable original presence. The basic dikes are described under 

 the following three types, diabase including olivine-diabase, camp- 

 tonite, and moncLiquite. 



The Diabase. 



The true diabase dikes are principally, if not quite entirely found 

 in the crystalline Archaean areas. They are thus on the west side 

 of the lake, and in several cases back in the mountains. Diabase 

 is the characteristic type of rock that forms the dikes which so 

 plentifully intersect the lenses of magnetite. 



The rocks exhibit, under the microscope, the characteristic ophitic 

 structure with occasional radiating arrangement of feldspars 

 (divergent-strahlig of Lessen). These have, at times, a dark core 

 which follows the outline of the crystal, and is, doubtless, an 

 altered portion stained by infiltration. The plagioclase was sepa- 

 rated from a coarse dike from near Port Kent, and afforded the fol- 

 lowing analysis: — 



I. II. At. Ratio. 



Loss 0.72 



SiO^ 57.82 58.38 .973 11 



AI./J3 28.16 28.43 .270 3 



CaO 7.72 ■ 7.79 .139 % 



Na.p 5.35 5.40 .087 1 



99.77 100.00 



From this it appears that the feldspar belongs in the andesite 

 series. Column II is recalculated to throw out the loss on ignition. 



The augite tends to become at times idiomorphic, and marks a 

 passage into the augite-camptonites. This tendency is wide-spread, 

 and indicates the close affinities of the two rocks. The augite 

 varies from pink to greenish. In the heavy residues obtained by 

 panning crushed material, some grains of hypersthene were found, 

 although none were ever noted in the slides. The discovery is 

 interesting in connection with its recently found presence in Triassic, 

 diabase.' Subordinate biotite is occasionally seen in scales with 

 magnetite, and is regarded as an original mineral.'' Irregular 

 grains of magnetite are abundant. 



The process of alteration seems to be the same wherever noted. 

 The bisilicates pass into chlorite, or into what in other cases appears 

 to be serpentine. The latter is a yellow or amber colored mass, 

 not always showing a very marked, aggregate character. It is 

 lacking in pleochroism and is not very strongly refractive. Some 

 dikes are in such an advanced stage of alteration ihat they present 



^ Campbell and Brown, Composition of certain Mesozoic Igneous Rocks from 

 Virginia, Bull. Geol. Soc. of America, 1891, p. 339. 



2 Wadsworth takes a diflerent view of similar biotite, regarding it as sec- 

 ondary, Bull, ii, Min. Geol. Surv. p. G5, and PI. iii. 



