MINERALOGY AND PETROGRAPHY. 1 

 Petrographical News.— Chrustschoff 2 has re-examined the rock 

 of Island Wallamo, in Lake Ladoga, Finland, that was first described 

 by Kutorga as a crystallized labrador-granite. Two varieties were 

 recognized by Chrustschoff, the one a dark-brown dolerite, composed 

 essentially of tabular plagioclase, idiomorphic pyroxene, olivine and a 

 little glass, with sanidine surrounding the plagioclase, and quartz crys- 

 tals embedded in micro-pegmatite, occurring in the interstices between 

 the plagioclase crystals. The second type is a dark-green diabase-like 

 rock, whose constituents are the same as those of the first-mentioned 

 rock. In this quartz and orthoclase are rare. These rocks are cut by 

 narrow dykes and veins of granophyre, without peculiar features. In 

 explanation of these phenomena the author states that the original 

 rock was an olivine diabase that had solidified, with the exception of 

 its glassy ground mass, when it was intruded by granophyre. The 

 acid magma partially dissolved the crystals and the unsolidified glass 

 of the intruded rock, and so produced an orthoclase quartz aggregate. 

 The basic plagioclase was corroded, and sanidine separated from the 

 mixture formed by its solution in the acid magma, while the remaining 



acid material cooled as granophyre. Deecke 3 gives a very detailed 



account of the geological and pet rographieal relationships of the gray 

 tufa of the Campagna, Italy, which he believes to be a product of the 

 volcanoes of the Phlegraean Fields. This tufa now consists of a col- 

 orless or pale yellow glass, in which are imbedded fragments an 1 crys- 

 tals of a very soda-rich sanidine, augite and biotite. The rock is thus 

 an augite-trachyte. An analysis of the crystals of sanidine gave : 



SiO, A1 2 0, FeO CaO MgO K,0 Na 2 Loss Total 

 63.79 20.87 1.09 2.06 .41 7.56 3.72 .42 99.92 



Besides the fragments of minerals there i 



are also found enclosed m 



the tufa pieces of augite-trachyte, pumice 



and c 



•bsidian, fragments ot 



hornblende-trachyte, and others of sedime 



n tan- 



rocks. A noticeable 



and very characteristic feature of the tufa 



that 



distinguishes it from 



others occurring in the same region, are tin 



B nun 



lerous geodes distnb- 



uted in great numbers through its mass. 



These 



contain a yellowish 



powder, consisting of sanidine, tufa-fragme 



nts a. 



idfluorite. They are 



. Bayley, Colby Univ 



