536 DESCRIPTIVE GEOLOGY. 



inates over feldspar. In general, there is more ortlioclase than plagloclase. 

 The series is particularly instructive as illustrating the breaking-up of the 

 schistose structure and assumption of granite forms by the development of 

 large feldspars, and by the compression which has been brought to bear in 

 the direction of the beds. There seems evidently to be some connection 

 between these two phenomena; for wherever the parallel structure of the 

 mica and hornblende is broken up into wrinkles and waves, there the feld- 

 spars assume greater importance, and reach the size of large peas. The 

 rock in such passages, in hand-specimens, is indistinguishable from a gran- 

 ite; and between this and the schist with parallel mica, in which no crystals 

 of quartz or feldspar interrupt the sheets of mica, there is every possible 

 transition. 



A very instructive exhibition of these changes is to be seen directly 

 east of the summit of Mount Bonpland, where an abrupt precipice of not 

 less than 1,000 feet falls off to the east. Across the face of this precipice 

 may be traced the parallel layers of the gneiss, which vary in tint from a 

 pale gray, almost white, to nearly black, according as the mica or horn- 

 blende predominates. Across these planes of bedding is an irregular cloud- 

 ing, which looks upon the surface of the precipice almost like the presence 

 of irregular intruded masses. Through these, however, the general lines 

 and shades of the gneiss beds are seen to run ; but the gneiss itself, within 

 the limits of these vertical cloudings, has lost much of its schistose character, 

 and is more granitic. 



The whole summit region is profoundly riven with vertical cracky par- 

 allel to the trend of the range, but to what depth is unknown. They are, 

 however, seen more or less distinctly throughout the whole of the eastern 

 precipitous face of the northern part of the range. Owing, no doubt, to 

 this series of vertical fissures, the summit rocks are very peculiarly weath- 

 ered into a series of tall pinnacles 50 or 60 feet high, witli large rounded 

 tops like mushrooms, formed of beds which have more successfully resisted 

 erosion. 



As already mentioned, this entire series is conformable, and dips to the 

 west, and overlies the White Cloud Peak body of granite, but overlies it 



