GROUNDMASS STRUCTURE. 247 
ably is secondary. Both the dikes have a miarolitic tendency, and these 
minerals have likely resulted from infiltration. 
Groundmass structure.—W hile this varies much in the dikes it may be 
summed up in the one word, trachytic. Four main types are presented : 
1. The structure is fluidal throughout, the lath-shaped feldspars hay- 
ing parallel arrangement and considerable interstitial quartz being pres- 
ent. This structure is limited to the more acid dikes, but only part of 
them possess it. 
2. Flow structure is only apparent around the phenocrysts, otherwise 
the structure is like one of the following. 
3. The structure is panidiomorphic granular, running into hypidio- 
morphic where the dark silicates become abundant. Even then these 
are largely included, and the majority of the contacts are of feldspar 
with feldspar. In all cases a few tabular, rudely idiomorphic feldspars 
occur, and may increase in abundance till they form most of the rock. 
4. A trachytic structure, of stout, interpenetrating feldspar laths, with 
a slight amount of interstitial quartz and orthoclase. 
FRESHNESS OF THE MATERIAL 
Considering the proneness of rocks of this type to decay, surprisingly 
fresh material can be obtained from these dikes. The ferromagnesian 
silicates, to be sure, are often altered, but the feldspars are quite fresh 
in the majority of cases. The rocks are far less affected than are the 
much younger so-called bostonites of lake Champlain. This may be in 
part accounted for by the less compact character of the bostonites, which 
probably solidified at less depth, and by the less resistant character of 
the enclosing rocks, but the situation of the earlier dikes is mainly re- 
sponsible for their present condition. They cut the low, outlying spurs 
of the Adirondacks, which felt the full force of glaciation and from which 
all material weakened by degradation must have been swept away. They 
also cut low, rounded, well polished exposures of the crystalline rocks, 
In nearly every case the postgiacial weathering is but skin deep, it being 
seldom necessary to remove an inch of surface material from the joint 
blocks to obtain fresh rock. In many of the associated diabases even 
the olivine is found largely unaltered, while farther back in the moun- 
tains it is much more difficult to obtain such material. 
SPECIFIC GRAVITY 
The following table of the dikes* shows the specific gravity of each 
*The numbers given the dikes are those applied in the field in the order of discovery and used 
in reporting to Professor Hall. The county has been used as the basis for numbering, but the 
great quantity of dikes renders the system cumbrous, and the township would be a better unit. 
XXXVII—Buut. Grou. Soc. Am., Vou. 9, 1897 
