Petrographical Notes on Rocks from Chelsea. 131 



primary constituent, and the whole, a sedimentary rock. We 

 should then have only to account for the presence of the 

 serpentine. 



(2) On the other hand, as the calcite does not offer such 

 evidence, the better explanation seems to be that the calcite has 

 been produced by the decomposition of some of the feldspars, 

 and, the serpentine having been derived from hornblende, (which 

 again, may have been primarily pyroxene), the original rock had 

 the composition of hornblende — (or augite) — diorite or syenite. 



This view is also in a measure corroborated by a reference 

 to the investigations of Mr. Ingall, (Can. Rec. Sci. Vol. VI, No. 

 2) in the pure limestone beds of the Laurentian system. 



Serpentine limestones are well-known members of the 

 Grenville series in Canada, and of its equivalent in the Adiron- 

 dack Mountain, region of New York. 



Their occurrence in the latter district has been fully 

 described by Prof Kemp, while the Canadian localities have 

 been made well-known by the officers of the Geological Survey 

 Department.* 



NO. 2. sillimanite gneiss. 



The essential constituents of this rock are feldspar, biotite 

 quartz and sillimanite ; the accessory, garnet and pyrite. 



The feldspar is much decomposed and seems to be mostly 

 orthoclase. No plagioclase can be surely identified. Biotite is 

 very prominent in the thin section. 



Both prismatic and basal sections are abundant. The 

 former show well-marked cleavage and parallel extinction, and 

 all have distinct pleochroism. 



Quartz is present in grains of various sizes several having 

 a rounded or somewhat elongated form. Some of the larger 



*See " Catalogue of .Stratigraphical Collection of Canadian Rocks," by \V. F. 

 Ferrier — 1893. 



pi' ij ' . i ..iiLi ii iii«.n. i i.i .11 .,. 1 1 ,.,j,.jL ! Ji.j, i .iiLiJiujiiiiyi4i i ii.jfc.n i M)Hiwji i iiuimmtuifc ' , ' j.t ' 



