REVIEWS — GEOLOGICAL SURVEY OF CANADA. 323 



These rocks, as a -whole, have very generally been called granite, by those 

 travellers who with little more than casual observation have described them, with- 

 out reference to geological considerations. The ruins of granite are known to 

 constitute an indifferent soil from their deficiency in lime, and hence an unfavora- 

 ble impression is produced in respect to the agricultural capabilities of any ex- 

 tended area, when it is called granitic. Such soils are however never wanting 

 in those essential elements the alkalies, Avhich are abundant in the feldspars of the 

 granite. 



In the Reports of the Survey, the Laurentian rocks have been described in 

 general terms as gneiss, interstratified with important masses of crystalline lime- 

 stone. The term gneiss, strictly defined, signifies a granite with its elements, 

 quartz, feldspar and mica, arranged in parallel planes, and containing a larger 

 amount of mica than ordinary granite possesses, giving to the rock a schistose or 

 lamellar structure. When hornblende instead of mica is associated with quartz 

 and feldspar, the rock is termed syenite, but as there is no distinct specific single 

 name for a rock containing these elements in a lamellar arrangement, it receives 

 the appellation of syenitic gneiss. 



G-neiss rock then becomes divided into two kinds, granitic and syenitic gneiss, 

 and the word gneiss would thus appear rather to indicate tbe lamellar arrange- 

 ment than the mineral composition. Granitic and syenitic gneiss were the terms 

 applied to these rocks in the first Reports ; but as granite and syenite are con- 

 sidered rocks of igneous origin, and the epithets derived from them might be sup- 

 posed to have a theoretical reference to such an origin of the gneiss, while at the 

 same time it appears to me that the Laurentian series are altered sedimentary 

 rocks, the epithets, micaceous and hoinblendic have been given to the gneiss, in 

 later Reports, as the best mode of designating the facts of mineral composition, 

 and lamellar arrangement, without any reference whatever to the supposed origin 

 of the rocks. When the general term gneiss therefore is used, it may signify both 

 kinds, or either ; and the epithets micaceous and hornblendic are applied to the 

 rock to indicate that the mica greatly preponderates or excludes the hornblende ; 

 or the hornblende the mica. 



In no part of the area included in this Report is hornblende completely absent 

 from the gneiss, and sometimes it predominates over the mica; hornblende con- 

 tains from ten to fifteen per cent, of lime, so that the ruins of the rocks of the 

 area, such as they have been described, whether gneiss, greenstone, syenite, or 

 porphyry, would never give a soil wholly destitute of lime. Of this necessary 

 ingredient, the lime-feldspars would be a more abundant source. Different species 

 of them from andesine toanorthite, may contain from about five up to twenty per 

 cent, of lime, and the range of those Canadian varieties which have been analyzed 

 by Mr, Hunt, is from seven to about fifteen per cent. The personal exploration 

 which is the subject of the present Report, has shewn, for the first time, that 

 these lime-feldspai-s occur in this Province, and pi-obably in other regions, in 

 mountain ranges, belonging to a stratified deposit, and not in disseminated or in- 

 trusive masses. The breadth of these displayed in the district examined, demon- 

 strates tiieir importance ; and the fact that the opalescent variety of labradorite 

 was ascertained by Dr. Bigsby to exist, in situ, on an island on the east coast of 

 Lake Huron, while the name of the mineral reminds us of its existence at the 



