[itfyusices of Lake JferiijiJircrufn/of/. — Mdrstcrs, Bl 
subjected to some eroding process. It seems probable tliat 
the more minute tlecks and shreds of biotite found in the 
ground mass may have been derived from tlie hirger individ- 
uals which originally may have been idiomorphic. Pleo- 
cliroism is especially prominent. Plates parallel with the 
cleavage show a bright brown to very dark brown or nearly 
black color, while sections across the cleavage exhibit lighter 
shades of brown with a yellow tinge, and upon rotating the 
stage becomes dark greenish yellow. The biotite is gener- 
all}' quite fresh but in a few instances a partial decomposi- 
tion to a reddish opaque mass was noted. Small amounts of 
magnetite are present with the biotite in addition to the fine 
dust noted in the quartz grains. 
Laniprophiire Dikes. 
While a few of the dark colored intrusives are undoubtedly 
diabase the majority of occurrences show various porphyrit- 
ic constituents wliich even by an examination of tlie hand 
specimens led me to anticipate the repetition of the same 
mineral mixtures (monchiquite group of Rosenbusch) as were 
found on the borders of lake Champlain to the southwest. 
As was expected microscopic examination reveals all the va- 
rieties of the monchiquite group except the biotite-araphibole 
mixture designated by J. F. Kemp as amphibole ouachitite.* 
Diabase. — The true diabases differ much from the typical 
diabases of the Triassic trap-ridges of the Hudson river or 
similar flows forming the north mountains of Nova Scotia. 
The dikes under consideration differ in containing a much 
smaller percentage of lath-shaped feldspar and a correspond- 
ingly large amount of augite, with occasional olivine pheno- 
crysts. Ophitic structure, so common in the Triassic diabases 
of Nova Scotia, is quite generally absent in thin sections. 
The augite proves to be the most prominent colored silicate 
present. It occurs in two generations, as phenocrysts and as 
irregular grains uuiking up a portion of the groundmass and 
partially filling- the interstices between the feldspars. The 
I)orphyritic form is well developed, showing in cross section 
the characteristic cleavage angle and crystalline form. In 
most instances they exhibit light yellow rims with slight 
*Ark.GtH)l. Survey, vol. ii, p. S9:5, 1890. Basic dikes outside of the 
sv(Miite an-as of Arkansas. 
