396 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



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

 logically a single character of the Quincy or Cape Ann gi'auites or 

 syenites. 



The specimen of " Exeter syenite " obtained from Prof. Hitchcock, 

 and now in the hthological collection at the Museum of Comparative 

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

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

 former appearing as well-striated plagioclase. From its resemblance 

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

 it belonged rather 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, that the rock is composed of plagio- 

 clase, a little orthoclase, augite, biotite, magnetite, apatite, and some 

 quartz. The augite is seen in places to have suffered alteration and to 

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

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

 amounts, and occupies the angular interspaces usually filled by the 

 base in such rocks when unaltered ; it is therefore probable that it has 

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

 silica after the other minerals had crystallized out. The plagioclase 

 proves, as determined by the method of Descloizeaux, to be labradorite. 

 This rock, therefore, is a gabbro, or a coarsely crystallized diabase, and 

 belongs rather to the so-called dorian rocks (basaltic) than to the gran- 

 ites. (See M. E. Wadsworth in Proc. Post. Soc. Nat. Hist., 1877, XIX.,. 

 pp. 217-238; Bull. Mus. Comp. Zool., 1879, V., pp. 275-287.) 



"We have here, therefore, three rocks, almost as different 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 which may be cited as illustrative of the value of 

 the lithological determinations of the Xew Hampshire Survey is that of 

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

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

 The locality is Piermont, N. H., and the region where this rock occurs is 

 described as being made up of alternating bands of whetstone slate (mica 

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



" there are five bands of hornblende schist and five of whetstone slate I 



am inclined to carry out the suggestion of a previous page, to the eff'ect that 

 much of this hornblende should be regarded as Huronian. In agreement with 

 this notion, it would form hummocks underlyhig the mica schists, probably 



