C. H. Warren — Miner alogical Notes, 119 



satisfactory explanation, however, can be given for the low 

 sesquioxides in Damour's analyses. The formulas which have 

 previously been assigned to these minerals are Fe 2 Pb 2 Si 2 9 and 

 Mn 2 Pb 2 Si 2 9 , respectively, with the ratio of Si0 2 : PbO : R 2 3 = 

 2 : 2 : 1, and, with the exception of Damour's analyses, it will 

 be seen from the following table of ratios that the agreement 

 is not very satisfactory. 



Si0 2 : PbO : R 2 3 . 



Author 2-06 : 2 : ISl 



Lindstrom I 1 2-34 : 2 : 1*22 



Lindstromll 2*18 : 2 : 1-10 



Damour 1*98 : 2 : 1*00 



Flink 2-36 : 2 : 127 



From the foregoing it may be seen that the analyses thus far 

 have shown a considerable variation, and with the exception of 

 Damour's analyses of kentrolite, they agree better with the 

 author's formula than with the one that has previously been 

 accepted. The author's formula for melanotekite (Fe 4 3 ) Pb 3 

 (Si0 4 ) 3 , is substantiated not alone by the analyses of the 

 mineral from Hillsboro, but it corresponds to a type of basic 

 silicate formula, which is quite common among minerals, 

 namely, three molecules of orthosilicic acid in which six hydro- 

 gens are replaced by the sexivalent basic radical (Fe 4 3 ) IV , and 

 six by three atoms of lead. Moreover, if the formula for 

 melanotekite is correct, the formula of kentrolite must be 

 similar (Mn 4 3 ) Pb 3 (Si0 4 ) 3 , for the two minerals have almost 

 identical crystallographic relations. 



2. Pseudomorphs after Phenacite, from Greenwood, Maine. 



The pseudomorphs, which are to be described, were discovered 

 by Mr. G. L. Noyes, of Norway, Maine, at a ledge in the town 

 of Greenwood, and were sent to this laboratory for identification. 

 The crystals are remarkable for their size, and are evidently 

 pseudomorphs, as the faces are rough, in places pitted, and in 

 the depressions perfectly developed quartz crystals are visible. 

 A thin section, when examined under the microscope, showed 

 that the material consisted of quartz and a mineral with a 

 foliated or mica-like structure. The color of the crystals is 

 white with a slight greenish cast. It is evident that the crystals 

 were originally attached, and they had been broken from the 

 matrix, so that they thus presented only a portion of the crystal 

 faces, and although the habit differs from that of phenacites 

 which have been described, the characteristic forms and the 

 peculiar symmetry of the crystals render it almost certain that 

 they must be pseudomorphs after that rare mineral. The 

 following forms were observed : 



