BASIC VOLCANIGS OF HEMLOCK FORMATION. 105 



Measiii'ements made on the feldspar microlites of the groundmass gave 17° 

 as the maximum extinction in zone perpendicular to 010. This angle points 

 toward the microlites being acid labradorite. The microlites thus seem to 

 agree essentially with the phenocrysts in composition. Flowage structure 

 around the phenocrysts is most distinctly shown by the arrangement of the 

 feldspar microlites. In one case in which the porphyritic texture and the 

 flowage structure are very good, secondary actinolite crystals have devel- 

 oped parallel to one another and parallel to the flowage direction, giving 

 the rock under the microscope a distinctly schistose appearance. 



Chemical composition. — In tlic preliminary article upon these Hemlock vol- 

 canics published in 1895,^ the occurrence of andesites as well as basalts was 

 mentioned. This determination was based solely on the microscopical study 

 of the rocks, and the rocks which were presumed to be andesites were those 

 porphyritic forms which have just been described. Since the publication of 

 that article the following analyses (Nos. 1 and 2) have been obtained of 

 the porphyritic rocks. The rocks selected for analysis were those which 

 appeared to be especially rich in feldspar, and, having a rather lig-hter color 

 than the others, seem to be somewhat more acid than the average. These, 

 it was thought, might have the composition of andesite. 



The comparison of series of rocks derived presumably from the same 

 magma is more profitable than the study of single analyses. This line of 

 investig-ation, as followed by Rosenbusch,^ Iddings,^ Lang,'' Broegger,^ 

 Becke,® and Michel L^vy,^ has been very fruitful. 



basalts in which it is present in very small quantity, filling irregular but iu general rounded areas 

 between the other constituents. It ■would, of course, be impossible to base the determination of the 

 former presence of leucite iu these pre-Cambrian rocks upon such scant evidence as has been obtained. 

 Still it is worth while to notice even such a doubtful indication of its former presence as has been 

 mentioned above. 



I Jour. Geol., cit. pp. 805-806. 



' Ueber die chemischen Beziehungen der Eruptivgesteine, by H. Rosenbusch : Tsch. Mln. u. Pet. 

 Mitt., Vol. II, 1889, pp. 144-178. 



s Origin of igneous rocks, by J. P. Iddings : Bull. Phil. Soc. Wash., Vol. XII, 1892, pp. 88-214. The 

 eruptive rocks of Electric Peak and Sepulchre Mountain, by J. P. Iddings: Twelfth Ann. Kept. U. S. 

 Geol. Survey, 1892, pp. 571-664. 



•■ Ordnung der Eruptivgesteine nach ihrem chemischen Bestand, by H. Otto Lang : Tsch. Min. u. 

 Pet. Mitt., Vol. XII, pp. 199-252. Beitrage zur Systematic der Eruptivgesteine, by H. Otto Lang : Tsch. 

 Min. u. Pet. Mitt., Vol. XIII, 1892, pp. 115-169. 



" Die Eruptivgesteine des Kristianiagebietes, by AV. C. Broegger. I. Die Gesteine der Grorudit- 

 Tinguait serie. II. Die Eruptionsfolge der triadischen Eruptivgesteine bei Predazzo in Siidtyrol. 

 Videnskabsselskabets Sknfter, I Mathematisk-naturv. klasse. No. 4, 1894; No. 7, 1895. 



6 Gesteine der Columbretes, by F. Becke : Tsch. Min. u. Pet. Mitt., Vol. XVI, 1896, pp. 308-336. 



'Note sur la Classification des Magmas des Roches firuptives, by A. Michel L^vy; Bull.de la 

 Soc. Geol. de France, .Sd ser., Vol. XXV, No. 4-. .July, 1897, pp. 326-377. 



