Decembek 13, 1895.] 



SCIENCE. 



823 



a perfect ellipse, about seven miles long in a 

 northeast-southwest direction, and four to five 

 miles in breadth. It is nearly surrounded by 

 hills of ' hornfels ' or metamorphosed clay slate, 

 whose elevation above the more easily weath- 

 ered granitite is well shown by the drainage. 

 To the southeast lies a portion of the Schlema 

 stock of turmaline granite, also bordered by its 

 metamorphic rim of hornfels. The metamor- 

 phic belt belongs to the non-fossiliferous Phyl- 

 lite series and higher, the Cambrian slates. The 

 alteration due to the intrusion of the granitic 

 masses is similar in both formations. The 

 highly crystalline zone next to the granite has 

 an average thickness of 300m., and the outer 

 zone of spotted schist varies from 450m. to 550m. 

 in thickness. These measurements are made 

 perpendicular to the original cooling surface of 

 the granite ; the many mining shafts in the 

 vicinity of Schneeberg afford accurate data for 

 such measurement, and show that the alteration 

 zones are determined solely by the position of 

 the granite, quite independently of the dip and 

 strike of the sediments. 



The Kirchberg granitite stock consists of a 

 coarse, porphyritic outer shell enclosing a some- 

 what later intrusion of finer grain ; the contact 

 of the two, however, shows that the older 

 magma was still partially fluid when the 

 younger was intruded. At the contact with 

 the hornfels the granitite often interpenetrates 

 the slaty folia in very fine veinlets showing ex- 

 treme liquidity at the time of intrusion. 



At the contact the hornfels contains musco- 

 vite, biotite, quartz, andalusite and magnetite ; 

 at a distance from the contact it becomes more 

 schistose in character, and greenish-black ob- 

 long spots appear, which are chiefly concretions 

 of carbonaceous pigment ; going further, the 

 spots disappear, but the slaty folia still retain a 

 crinkled apjiearance, until finally the unaltered 

 clay slates are reached. The unaltered phyllite 

 and the andalusite hornfels show great simi- 

 larity in chemical composition, indicating mo- 

 lecular rearrangement rather than actual acqui- 

 sition of new material. 



The Pirna area lies southeast of Dresden, be- 

 tween the great Lausitzer granite stock on the 

 northwest and the gneiss of the Erzgebirge on 

 the southeast. A concise summary of the geol- 



ogy of this contact series is given by R. Beck, 

 Tschermaks Mineralog. und Petrog. Mitthei- 

 lungen XIII-4-p. 290, 1893. 



Southwest from the town of Pirna the altered 

 sediments of Cambrian, Silurian and Devonian 

 age lie in apparently conformable succession, 

 highly inclined and striking northwest. Vari- 

 ous granitic masses cut them, producing dif- 

 ferent metamorphic changes, according to the 

 nature of the rock affected. 



The Lausitzer Granite consists chiefly of an 

 oligoclase-quartz-biotite granitite, which in the 

 vicinity of Dohna is replaced by a micaceous 

 granite. A syenite occurs further south in ob- 

 long masses parallel to the strike of the sedi- 

 ments, and this varies locally to hornblende- 

 granitite in one case and to quartz-angite-diorite 

 in another. In the southeast part of the area 

 occurs a large granitite stock near Markersbach, 

 characterized by pnevimatolytic phenomena and 

 intersected by veinlets which contain cassiterite, 

 topaz, blende, zinnwaldite, turmaline, fluorite, 

 etc. Near it the turmaline-granite of Gottleuba, 

 which shows much kataclastic alteration, occurs 

 iu a number of long lenticular masses which 

 lie in general parallel to the strike of the asso- 

 ciated sedimeutaries. This indicates that dy- 

 namo - metamorphism played a part in the 

 changes wrought in this basin, though Beck con- 

 siders them chiefly contact phenomena, the 

 dynamic action having taken place long after 

 the early igneous intrusions and affecting both 

 granites and stratified rocks alike. 



The contact metamorphism observed is as 

 follows: 



The Phyllites are altered into spotted schists 

 and andalusite-hornfels as in the Kirchberg area 

 and elsewhere. Chlorite gneiss is altered to 

 biotite gneiss, a somewhat unusual process (see 

 Beck, 1. c. and also C. Callaway, Geol. Mag. 

 (3) 10. 535-538. 1893). 



Silurian clay slates are altered to ' knoten ' 

 schists and nearer the contact to Cordierite- 

 hornfels. Contrary to the rule observed in the 

 phyllites, the ' knoten ' in thin section are spots 

 of less pigment than the mass of the schist, and 

 in the hand specimen appear as tiny blisters or 

 nodal points. A carbonaceous lydite or sili- 

 ceous schist becomes graphitic near the granite 

 contact. 



