210 MINERALOGY AND LITHOLOGY. 
and plagioclase, apatite, etc., as accidental constituents. The character- 
istic feature of gneiss is, that it possesses a stratified or laminated appear- 
ance, on account of the parallel arrangement of the individual crystals of 
the essential accessory. Like granite, gneiss is sub-divided, according 
to which accessory it contains, into as many kinds as are characterized 
by different minerals. The difference between gneiss and granite being 
therefore merely structural, all that has been said of granite may, with 
slight modification, be true of gneiss; and hence, though it is one of the 
commonest rocks in New Hampshire, and geologically one of the most 
important, it may be briefly treated here. 
There are many structural varieties of gneiss. Sometimes the acces- 
sory, which is usually mica, lies in flat planes, and the rock appears to be 
uniformly foliated or stratified, while in other cases the mica is in most 
irregular wavy or twisted layers, of most complex structure; and many 
names of varieties have been suggested by peculiar characters of this 
nature. 
The cause of the lamination has been made a study by many eminent 
geologists, and it is by no means certain that in all cases it is to be re- 
ferred to the same one. There are some who still hold that the min- 
eral arrangement is caused by the direct deposition of the minerals on 
the bottom of bodies of water which hold the constituents in solution; 
but the microscopic characters of the minerals and their inclusions, 
being the same as those of granites, point to formation and crystalliza- 
tion under enormous pressures and at elevated temperatures, and ren- 
der this supposition untenable. Again: it is supposed that gneisses 
are made by the recrystallization of such sedimentary beds as argillites 
or clay slates, under the same influence of heat and pressure as made the 
granite, but that these influences were milder in their action, and the 
present condition is the result of the original stratification of the deposits. 
The plane of lamination of the gneiss is often found to be conformable 
with the strata of a region, and it cannot be denied but that original 
lamination may, in some cases, have had its influence. The third sup- 
position is, that the stratified structure was induced by pressure, and by 
retarded movements in the once plastic mass. This theory was first 
advanced by Scrope,* in 1825, to whom the idea was suggested by the 
*Scrope. Craters and Lavas. Your. Geol. Soc., 1856, p. 346. 
