76 



cent, of silica ; hence, whether Huronian or not, it is probably 

 a true felsite. 



The compact, non-porphyritic, red petrosilex occurring in 

 Saugus — the so-called " Saugus jasper" — is well known. 

 It covers but a small area, and is found on the south side of the 

 Saugus River, a few rods north-east of the station in Saugus 

 Centre. An analysis, made many years ago by Dr. C. T. Jack- 

 son, showed that this rock is essentially feldspathic, and not, in 

 any proper sense, a jasper ; and its superior fusibility points to 

 the same conclusion. Having been unable to find any record of 

 Dr. Jackson's analysis, I will append the results of a single 

 analysis made by Mrs. Crosby : silica, 81.1 per cent. : sesqui- 

 oxides of aluminum and iron, 13.35 per cent. ; alkalies, not 

 determined. These data make it clear that this interesting rock 

 is not only feldspathic, but a veritable petrosilex. Dr. Jackson 

 gives the following analysis 1 of a " green petrosilex " (probably 

 compact) from Melrose : — 



Silica, 



86.00 



Oxides iron and alumina, 



2.00 



Lime, . 



1.12 



Magnesia, . 



1.10 



Water, . 



1.50 



Alkalies, . 



8.28 



100.00 



I have not seen this rock, and do not know its exact locality, 

 but its composition is so anomalous for petrosilex that I con- 

 clude it must be an impure quartzite. The large percentage of 

 alkalies in an apparently uncombined state, however, suggests 

 an error in the analysis. 



The great mass of the petrosilex in Lynn and Southern Sau- 

 gus, the characteristic variety, is little porphyritic, frequently* 

 quite compact, and usually of some shade of red, brown, or 



1 Proc. Bost. Soc. Nat. Hist., in., 84. 



