2IO 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. Ceol. Soc, 1856, p. 346. 



