1893.] : Mineralogy and Petrography. 899 
same consanguinous group as do the Cordilleran rocks of Mexico and 
the United States, and their nature indicates that the magma produc- 
ing the Andes types has not yet become as highly differentiated as that 
which yielded the corresponding volcanics in North America. 
Basalts and Trachytes from Gough's Island.— Pirsson* has 
examined some pebbles gathered from the beach of Gough’s Island in 
the South Atlantic. He finds two of them consisting of basalt, and the 
others of trachyte glass and tuff. The glass is a pitchy-black mass, 
filled with small pores and marked here and there by a phenocryst of 
plagioclase. In thin sections it appears as a brown unaltered isotropic 
substance containing magnetite, apatite, olivine and saidine phenocysts 
and mierolites of the last-named mineral. An analysis of the rock 
gave: 
SiO, TiO, Al,O, Fe,O, FeO MnO Mgo CaO Na,O K,O H,O Total 
61.22 .42 18.01 1.32 4.51 tr. .44 1.88 6.49 5.93 .46—100.68 
Density = 2.210. The rock is thus shown to be unquestionably a 
trachyte in spite of the fact that it contains occasional olivines. The 
mineral evidently crystallized in an early stage of the rock’s history, as 
all its grains have been subjected to magmatic resorption. 
The Origin of the Gneisses of Heidelberg.—In gneisses occur- 
ring in the region northwest of Heidelberg, Osann' finds lenticular 
masses of graphitic and apatite schists, and therefore concludes that 
the gneisses are of sedimentary origin. The rocks do not possess the 
true gneissic foliation, since their feldspar, quartz, etc., do not show a 
sequence in origin, nor do their micas exhibit the pressure phenomena 
usually observed in the micas of other gneisses. Their structure is 
described as the “ hornfels structure " which is characteristic of contact 
products. The graphitic schists consist principally of quartz, musco- 
vite, graphite and flecks like the “ Knoten” of contact rocks, which 
are formed by the aggregation of plates of a green micaceous substance. 
The apatite schist is composed of 55% apatite, 43% quartz, and 2% of 
graphite, tourmaline and rutile. An analysis gave: CaO = 30.22 ; 
P, 0, = 22.86; F == 2.16; Insol. — 43.52. 
Petrographical News.—Retgers* communicates in a few brief 
notes the results of his examination of rocks collected in southern 
* Amer. Jour. Sci., XLV, 1893, 
p. 380. 
7 Mitth. gross. Bad. geol. Landesanst. Bd II, p. 372. 
* Neues Jahrb. f. Min., ete., 1893, I, p. 39. 
