PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 567 



Porphyryte. Anclcsyte.] 



No. 844. PORPHYRYTE. 



From a dike in the Motley granite. 



Ref. Annual Report, xiii, page 39. Same as Museum No. 2595. 



A fine-grained, green rock, looking like some of the more massive green- 

 stones of the Archean. 



Nic. The section is composed largely of fine-grained, green hornblende and 

 small plaaioclase laths. The plagioclase also occurs in laths of a size three or four 

 times that of the general grain of the rock. There are some rounded areas which 

 seem to represent some original ferromagnesian phenocrysts now largely altered to 

 hornblende. This was perhaps hypersthene. 



One section. 



Aye. Dike cutting Archean rocks. u. s. o. 



No. 845. ANDESYTE. 



From a dike in the granite at Sauk Rapids. 



Ref. Annual Report, xiii, page 39. Same as Museum No. 2122. 



Meg. Hand specimen not found. 



Mic. The section shows plagioclase laths, small, but of varying sizes, embedded 

 in an almost opaque groundmass. Where the slide is thin this groundmass is of a 

 yellowish color and under a high power and crossed nicols breaks up into an aggre- 

 gate of minute particles. The nature of these particles is not clear. The ground- 

 mass probably was originally glassy and is now devitrified. Scattered through the 

 section are the remains of some old phenocrysts which show embayments of the 

 groundmass. These phenocrysts are now completely altered and their place is filled 

 with an indistinct, fine-grained aggregate consisting largely of flakes of muscovite 

 and chlorite and minute grains of magnetite. One thick section. 



Age. Dike cutting Archean rocks. u. s. o. 



No. 846. ANDESYTE (?) 



" Amygdaloidal dike rock, Maine Prairie." 



Ref. Annual Report, xiii, page 39. Same as Museum No. 2123 (Annual Report, vii, page 51). 



Meg. Hand specimen not found. 



Mic. The section shows a groundmass which is quite fine grained. It is 

 composed of feldspar, green hornblende and biotite. The feldspar is largely plagio- 

 clase in the form of imperfect laths. Throughout the section are a few more or less 

 rounded phenocrysts of plagioclase and of quartz, and there is one large area of 

 considerable size, composed of a number of feldspar grains. The rounded nature of 

 the phenocrysts probably caused the rock to be called an amygdaloid, but the slide 

 does not show conclusively that it is an amygdaloid. One section. 



Aye. Dike cutting Archean rocks, u. s. G t 



