VOLCANICS OF THE MICHIGAMME DISTRICT. 813 
them the secondary products calcite, epidote, muscovite and 
chlorite, and by their alteration alone could be readily dis- 
tinguished from the clear quartz. Moreover, the quartz and 
feldspar can be distinguished at a glance in certain sections by 
the occurrence of the aureoles around the quartz. In no case 
was an aureole observed to occur around a feldspar. 
No ferro-magnesian minerals are found in the porphyry, nor 
is their former presence indicated by any secondary product. 
The secondary minerals contained in the porphyries are chlorite, 
calcite, epidote, muscovite and biotite. 
Fic. 1.— Micro-drawing of quarts, multiplied 47 diameters, showing cleavage. 
The microscopic character of the dense groundmass varies 
according to the mode of association of its two chief elements,. 
the quartz and feldspar, and we thus get the varieties of prophyry 
named after the resulting structure. 
The commonest variety is the quartz-porphyry with micro- 
granitic groundmass (Porphyre granulitique of Michel-Lévy). 
A second variety is the quartz prophyry with micropoikilitic’ 
groundmass. The microgranitic structure is too well known to 
warrant a description of it here. The micropoikilitic structure 
which is present in the porphyries seems, however, to be a phase 
which is somewhat different from any thus far described or 
figured, and therefore deserves to be described more at length. 
tJ. P. Ippines: The Eruptive Rocks of Electric Peak and Sepulchre Mountain.. 
12th Ann. Rep. U. S.G.5S., p. 589. 
G. H. WiLLiAmMs: On the Use of the Terms Poikilitic and Micropoikilitic in 
Petrography. Journ. of Geol., Vol. I., No. 2, pp. 176-9. Feb.—Mar., 1892. 
