430 A FOPULAR EXPOSITION OF THE 
less coarsely-vesicular structure, or contains oval or other shaped 
cavities usually filled with cale-spar, amethyst-quartz, agates, various 
zeolites, &c., the rock is called an Amygdaloid, or Amygdaloidal 
Trap. Numerous examples occur in the northern district of Lakes 
Huron and Superior; and the agates of Michipicoten Island and 
other localities of this region, are derived from the disintegration and 
washing away of the amygdaloidal traps in which they were originally 
enclosed. 
The greenstones, or diorites, occur under the same conditions as the 
traps. Compact and amygdaloidal varieties are common about Lake 
Huron, &c.; and Sir William Logan, in his Report for 1853, has. 
described the occurrence of a columnar greenstone in the Township 
of Grenville, Argenteuil Co., C. E. In some greenstones, the com- 
ponent minerals, feldspar and hornblende, become individually per- 
ceptible. This variety might be called, indifferently, a granitic 
trachyte, or a granitic diorite, and placed in either of these groups.* 
A latitude of this kind, in the classification of these eruptive rocks, 
is unavoidable. Their frequent transitions and irregularities of com- 
position, render the drawing of very definite lines a complete im- 
possibility. For this reason, the attempt to frame a number of so- 
called species out of the trappean and other eruptive rocks, and to: 
bestow upon these distinct names, becomes both useless and unphilo- 
sophical. j 
Finally, it may be observed, that many varieties of trap and green- 
stone are very subject to decomposition, yielding soils of much fer- 
tility. By weathering, they become mostly dull-grey, brown, or red. 
5. Serpentines—The rocks of this series are essentially hydrated 
silicates of magnesia. They consist, strictly, of varieties of one 
mineral substaxce, serpentine. (See above, p.159.) Their colour is 
somewhat variable, but chiefly green, brown, reddish, or greenish- 
grey—these tints frequently occurring together in veins and patches. 
They are more or less soft and sectile, and somewhat granular or com- 
pact in structure; forming dykes and irregular masses, although 
comparatively of rare occurrence as eruptive rocks. Most serpentines 
are found in large beds, and are, evidently altered sedimentary deposits 
or metamorphic rocks, but undoubted instances of eruptive serpen- 
* If minute distinctions be advisable, the term granitic trachyte might be restricted to- 
such of these rocks as contain orthoclase or potash feldspar, whilst those in which triclinic: 
feldspars are present might be called granitic diorites ; but it is not always possible to carry: 
out these distinctions. 
ee ae 
