390 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



greenish micaceous and bornblcndic schist and gneiss, not having litho- 

 logically a single character of the Qnincy or Capo Ann granites or 

 syenites. 



The specimen of *' Exeter syenite" obtained from Trof. Hitchcock, 

 and now in the hthoJogical collection at the Musciu:n of Comparatiyc 

 Zoology, is unlike either of the preceding rocks. Macroscopicahy it 

 appears to be composed of feldspar and biotite, a large proportion of the 

 former appearing as well-striated plagioclasc. From its resemblance 

 to rucks of similar character occurring in Maine, it appeared likely that 

 it belonged rathei' to the basalts than to the granites; in other words, 

 that it was a basic instead of an acidic rock. Microscopic examination 

 of a thin section showed, in fact, tliat the rock is composed of plagio- 

 clasc, a little orthoclase, augite, biotitc, magnetite, apatite, and some 

 quartz. Tlie augite is seen in places to have sufFcred alteration and to 

 be partly changed to biotite, a common occurrence, rendering it proba- 

 ble that all the biotite is of secondary origin. The cpiartz is in small 

 amounts, and occupies the angular interspaces usually filled by tJie 

 base In sucli rocks when unaltered ; it is therefore probable that it has 

 eitlior replaced the glass in the process of alteration, or was itself residual 

 silica after the other minerals had crystallized out. The plagioelase 

 proves, as determined by the method of Descloizeanx, to be labradoritc. 

 Tliis rock, therefoi'c, is a gabbro, or a coarsely crystallized diabase, and 

 belongs rather to the so-called Xorian rocks (basaltic) tlian to tlie gran- 

 ites. (Sec ^I. E. Wadsworth in Proc. Best. Soc. Nat. Hist., 1877, XIX., 

 pp. 217-238; JhdI. M.US. Comp. Zook, 187D, V., pp. 275-287.) 



"We have here, therefore, tln-eo rocks, almost as difTercnt from each 

 other as it is possible for rocks to be, placed together as alike; and, on 

 this presumed analogy, practical advice in regard to exploration for min- 

 ing purposes is based. 



Another instance wJiich may be cited as illustrative of the value of 

 the lithological determinations of the isew Hampshire Survey is that of 

 No. 71 of tJic Preliminary Catalogue, there called hornblendite, and No. 

 221 of Hawes's Catalogue, where it is designated as hornblende schist, 

 'i'he locality is Piermont, N. IT., and the region where this rock occurs is 

 desenl)cd as being made up of alternating bands of whetstone slate (mica 

 seliist) and hornblende (hornblende schist). Prof. Hitchcock states that 



"there arc five bands of hornblende schist and five of whetsiouc sluic I 



am inclined to carry out the suggestion of a previous ])age, to the eifcct that 

 ]nuch of tliia hornblende should be regarded as Haronian. In iigi'cenient with 

 thiy notion, it would form hummocks underlyuig the mica schists, ]_irobab]y 



