:52 The American Geologist. -July, i89:« 
pasl summer collected specimens of several dikes from the 
vicinity of Lyon mountain, and ;i microscopic examination 
of the nicks shows them to vary somewhat from the types 
they describe. 
Red Laurentian gneiss tonus the country rock, although 
gray is also common; a few miles south, light and dark green 
noritc appears almost as abundant. 
The dikes occur on the west side of Upper Chateaugay 
lake. Three of them intrude through the gneisses along the 
lake front, and the rest occur within 200 rods of the shore, in 
Franklin county. Their positions are shown in the accom- 
panying sketch of the lake. The contours in the sketch were 
not accurately run. but are introduced simply to show the 
hilly nature of the region. For some distance north of In- 
dian point, the high knobs reach and form the water front, 
and dikes 2 and X cut through the rocky face of the bluff. 
The rest of the shore is low. and at its southern or upper end. 
is quite marshy. 
The country is heavily wooded and few outcrops of rock 
occur, but the presence of dike boulders scattered here and 
there on the surface, seems to indicate that the country is as 
abundantly seamed with these igneous intrusions as the re- 
gion of lake Champlain. 
The dikes are all narrow, from 1 to 8 feet, and have a 
general strike east ami west ; a direction that is characteristic 
of the dikes of this whole region. The three dikes on the 
shore of the lake will be described more in detail as the rest 
do not deviate from them in character. 
Two of the kinds of dikes described by Kemp and Marsters, 
occur; namely, the porphyry or trachyte, or so-called bos- 
tonite, and the olivine diabase, but notable differences in 
character exist. 
The rocks of these dikes are. chemically and structurally, 
of two greatly contrasted kinds. Dike 1. intruding through 
the gneiss at Indian point, is porphyritic and highly acidic 
It is a dark colored porphyry of fine matrix and phenocrysts, 
'.] to 8 mm. in size, of red orthoclase feldspar. 
Under the microscope the trachytic structure is well -ecu. 
The ground mass is noncrystalline and made up of small 
idiomorphic crystals of feldspar. The large phenocrysts of 
