Genth.] 40 [Oct. 2, 



10. Alteration of Magnesian limestone from Berks Co., Pa. 



The magnesian limestones in the neighborhood of Reading, Berks Co., 

 Pa., at Fritz's island, and about two miles east of Fritztown, two miles 

 south of Sinking Spring, at the Wheatfield and Ruth's Mines, frequently 

 undergo very interesting changes. 



Evidently by the infiltration of siliceous waters the magnesian limestone 

 is decomposed and from the magnesium carbonate, deweylite and serpen- 

 tine are formed, while another portion of the magnesia separates as bru- 

 cite ; the calcium carbonate crystallizes, both in the form of aragonite, in 

 small acicular crystals and radiating columnar masses, and in the form of 

 calcite, in crystals and coarse granular masses. 



Some of these minerals have already been described by E. F. Smith, 

 jointly with D. B. Brunner and J. Schoenfeld. I am indebted to Prof. 

 Brunner and Dr. Schoenfeld for numerous specimens of these interesting 

 occurrences. 



Brucite. — At Fritz's island brucite occurs in several varieties. 



a. In coatings of indistinct crystals 3-4 mm in diameter and crystalline 

 masses upon a granular limestone, largely altered into serpentine. It is 

 colorless in thin laminae and shows the characteristic pearly lustre. There 

 is also, on some portions of the limestone, a thin, white coating with 

 slight silky lustre, which may be brucite ; analysis a\. 



This brucite has already been analyzed by E. F. Smith (Am. Chem. 

 Journ., v, 281), whose analysis I give for comparison, a 2. 



b. A second variety is found in thin seams from 4 to 15 mm in thickness. 

 I could not observe any crystals but masses which are highly crystallized, 

 eminently showing the basal cleavage, but always in curved surfaces. It 

 has a slightly brownish-yellow tint, and on ignition becomes dark brown 

 from the oxidation of the considerable quantity of manganous oxide which 

 it contains. Spec. grav. = 2.382. From the analysis it will be seen that in 

 its composition it stands between pure brucite and Igelstrom's mangan- 

 brucite, although not so rich in manganous oxide, the latter containing as 

 much as 14.16 per cent. 



c. The brucite from near Sinking Spring, as Dr. Smith states, occurs in 

 thin colorless laminae in thin seams in the limestone, but also in silky fibrous 

 masses or even pulverulent, with but a faint silky lustre. Dr. Smith has 

 observed the fine silky fibres, but, not having had a sufficient quantity for 

 analysis, mistook them for hydromagnesite. The brucite is associated 

 with deweylite, coarse grained calcite and aragonite, in dolomite. 



I have analyzed a perfectly pure piece of the silky fibrous brucite, 

 which weighed nearly one gram, c 1, and for comparison give E. F. Smith's 

 analysis of the laminated mineral, r 2, from the same locality : 



