Z. V. Pii'sson — Geology of Conanicut Island. 375 



The rock is in that first stage of alteration of a lamprophyre 

 which may be called the carbonate stage ; it effervesces when 

 placed in acids, but the second stage of alteration in which 

 these rocks lose their carbonates and pass into masses of impure 

 clay, and which may be termed the kaolin stage, was nowhere 

 observed in either dike. The chemical composition is given 



Si0 o _ 46-11 



TiO\ -84 



X _: -97 



Al o 3 14-75 



Fe!,0 3 2-20 



Feb 4-51 



MnO tr. 



MgO 5-73 



CaO 7-82 



Na T29 



K a O 3-84 



ECO 2-90 



C0 2 7-32 



S 1-37 



Total ___ 99-65 



= S -33 



99-32 

 Sp. G. _ 2-904 



in the above analysis. The amount of C0 2 shows the con- 

 siderable amount of carbonates present and the sulphur that 

 of the pyrite. The symbol X refers to the oxides of those 

 earths which after fusion with HKS0 4 do not dissolve in cold 

 water. It is probably more or less impure with zirconia, 

 titanic oxide, etc. Xo attempt was made to determine it.* A 

 comparison of the chemical composition of this rock with that 

 of minettes from other parts of the world shows a striking 

 agreement with some of them as given in Roth's Tabellen. 



The same rock in other portions of the dike exhibits the 

 effects of pressure. The mica leaves are bent and curved, 

 they begin to disappear and their place is taken by a cloudy 

 decomposition product which shows perfectly the form and 

 cleavage of the original biotite crystal. All grades of transi- 

 tions may be traced from the unaltered crystals to those com- 

 pletely changed. On examining this cloudy alteration product 

 with the highest powers it is found to be full of minute rutile 

 needles and prisms. While other effects of pressure are 

 clearly visible, this is one of the most noticeable in this dike 



* Conf. Williams' Igneous Rocks of Arkansas, Ark. Geol. Surv. Ann. Rep., 

 1889, vol. li. p. 226, who gives full literature, etc. 



Am. Jour. Sci.— -Third Series, Vol. XLVI, No. 275.— Nov., 1893. 

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