THE MUSEUM. 



27 



are not known. Judging from the 

 bowlders which the inland ice bring 

 out to its border the interior of Green- 

 land opposite this region must be oc- 

 cupied by crystalline rocks, probably 

 of Archaen age. The specimens 



brought back from Cape Farday and 

 Clarence Head by the party that visit- 

 ed Ellesmereland show that sandstone 

 and shales, as well as granite and ig- 

 neous rocks, occur there also. 



Some Common New England 

 Rocks. 



One of the most common rocks 

 throughout New England is Mica 

 Schist. It is probably next to gneiss 

 the most abundant rock It is chiefly 

 composed of mica, but has thin layers 

 of glassy quartz, which are short and 

 overlapping. While it usually con- 

 sists of mica and quartz, it may be 

 composed of mica alone, or kaolin 

 and clay, sometimes take the place 

 of the quartz. In this case, however, 

 the mica is usually very fine, and the 

 rock is what would be called clay 

 slate. Also when the mica becomes 

 deficient, the substance passes into an 

 ordinary quartzite. Quite frecjuently 

 we find feldspar present in the mix- 

 ture. There is probably no other 

 rock that contains such a large variety 

 of beautiful accessory minerals as 

 mica schist. This makes it one of 

 the most interesting and attractive 

 rocks for the mineralogist. Also, few 

 rocks are so distinctly stratified; and 

 the stratifications can- usually be ob- 

 served in hard specimens. The mica 

 may be either muscovite or biotite, or 

 both, but the former is most common. 

 There is probably no other rock that 

 shows a greater variation in the per- 



centage of silica which it contains as 

 mica schist, for as above noted, we 

 find it from almost all mica to almost 

 all quartz. A rock that is closely re- 

 lated to mica schist is hydromica 

 schist, in which the ordinary anhy- 

 drous mica is replaced by dydromica. 

 It is easily distinguished from mica 

 schist by being somewhat softer, less 

 harsh to the touch, and less lustrous. 

 Gneiss, is the most important of all 

 rocks. It probably forms half of New 

 England and a very large propcrtion 

 of the earth's crust. The name (pro- 

 nounced the same as nice) is known to 

 have orginated among the Saxon min- 

 ers, but its precise derivation is lost 

 in obscurity. It is usually composed 

 of several minerals, the most common 

 of which is pink feldspar-orthoclase. 

 By close examination we find also 

 small quantities of quartz, which 

 usually fo/ms in small irregular, glassy 

 grains, entirely devoid of cleavage and 

 scratching glass easily. On weath- 

 ered surfaces, we find the orthoclase 

 soft and chalky, while the quartz 

 remains clear and hard. It 



has usually been considered that the 

 three main constituents of gneiss 

 was orthoclase, quartz and mica,, 

 but it is now recognized that he may 

 have true gneiss without any mica, 

 and that the latter may be represented 

 by hornblende. When the gneiss is 

 composed of quartz and orthoclase, it 

 is called ■binary gneiss. When it con- 

 tains mica in addition to the above, 

 we call it micaceous gneiss, and of 

 hornblende, hornblendic gneiss. How- 

 ever as noted above, orthoclase usually 

 comprises one-half of the rock. In 

 addition to the above minerals, we 

 frequently find as accessory minerals. 



