148 THE PENOKEE lEON-BEAEIKG SERIES. 



first phase prove microscopically to differ from those of the second in con- 

 taining- a re'latively large proportion of fine micaceous particles, these in- 

 gredients, including biotite, chlorite, and sericite, occurring either singly or 

 together. A still further increase in these ingredients, accompanied by an 

 increasing fineness in grain, gives rise to the third phase; while a still 

 greater increase in fineness of grain, along with an increase in the amount 

 of clayey material from the decomposition of the feldspar, leads to the 

 argillaceous slates or shales of the seventh variety. The microscopical study 

 shows further that the difference Ijetween these several })hases, so fai as 

 they are not the result of unusual conditions, as in the case of the cherty 

 and magnetitic l)reccias above mentioned, are almost entirely dependent 

 upon the original proportions and degrees of fineness of the two main frag- 

 mental constituents; that is, the quartz and feldspar. Some of these mica- 

 ceous ingredients are taken to be of fragmental origin. This is particu- 

 larly true of some of tlie large flaked sericite or muscovite, but in the main 

 these materials, including also kaolinite, appear to have resulted from a 

 decomposition of the feldspathic particles. This decomposition was accom- 

 panied by the separation of a secondary silica, which is now apjjarent in 

 most of the thin sections in the shape of a minutely crystalline quartz. 



Disregarding the special and rarer jihases, the important kinds may be 

 microscopically listed under the following heads: Chlorifk and Jnotitic 

 quartz-sMcs ; feldfipatl/ic (imtrtz-nlutfii; vitreous quarUHc ; sandstone ; novuetdite ; 

 arffiUaceoiis slates. These are variously interstratified with one another. 

 However, the vitreous quartzite layer is a persistent element in the stratig- 

 raphy, composing the u})permost })art of the fonnation wherever it is 

 exposed. The conglomerates referred to (Nos. 8, 0, and 10,) lie always at 

 or near the base of the formation, their peculiarities having been caused by 

 proximity to the underlying rocks ; but exposures of this kind are not suffi- 

 ciently numerous to demonstrate that such rocks are a persistent element in 

 the stratigraphy, although this is quite probable. At one place a vitreous 

 quartzite is at or near the base of the series. However, while no further 

 definite suVjordinate arrangement is observable in the cross-section, a change 

 in character is to l)c noted as the belt is followed from west to east, the 

 biotitic phase (Nos. 1 and ,'},) predominating in the western portion of this 



