Pirsson and Pice — Geology of Tripyramid Mountain. 277 



will be given later. With this preliminary statement the 

 details of the local geology follow. 



The Granite. — In the western half of the mapped area, 

 wherever we have been able to find the underlying country 

 rock exposed, it proves always to be of granite. Thus in the 

 "Ledges" above the hotel at "Waterville and along the ridge; 

 in Snow 's Mountain ; in the cascades on Cascade Brook ; in 

 the areas of bed-rock exposed in the lower course of Slide 

 Brook, as at Norway Rapids ; in the exposures at the Scaur 

 and in the Flume this is always the case. We infer from this 

 that the western part of the mapped area is underlain by 

 granite. This is only part of the great granite mass which 

 extends northward into the peaks of Osceola and Kancamagus, 

 southward into Sandwich Dome as far as Noon Peak and Jen- 

 nings Peak, and westward to the base of Mount Tecumseh* 

 where it is succeeded by mica schists not yet eroded. It is a 

 part ©f what we consider the great batholith which forms this 

 southern part of the White Mountains, and it seems to us a 

 reasonable conjecture that upon its descending southern slope 

 the mica schists still rest in irregular manner, and represent the 

 upturned beds, which, by folding, metamorphism, and the intru- 

 sion of the granite, have been converted into gneisses and schists. 



The granite itself is of a common type, a coarse-grained 

 mixture of flesh-colored alkalic feldspar and gray quartz with 

 a little biotite, and it is usually more or less altered in the visible 

 exposures. In places, as at .Norway Rapids, the granite is por- 

 phyritic with larger distinct phenocrysts of orthoclase feldspar, 

 and in Snow 's Mountain to the southern edge of the map and 

 beyond this character is persistent. Hitchcock on his geolog- 

 ical map makes a distinct boundary between the two varieties 

 of granite, but we do not feel that the amount of evidence we 

 could obtain warrants this. We do not know whether the por- 

 phyritie variety is a textural phase of the batholith, or whether 

 there are two granites of different periods of invasion present, 

 and, if so, which is the younger. Either assumption appears a 

 possible one. On the eastern edge of the area covered by the 

 map Prof. Huntington found that the falls on Sabba Day 

 Brook are cut in granite of the "Conway" type and that this 

 is the " common rock of the country." It extends upward for 

 a mile and is succeeded by gabbro, as discussed later. From 

 this survey then it appears that the general mass of Tripyra- 

 mid is known to be surrounded on all sides by granite, except 

 on the southeast, where possibly there may be a continuation 

 of its igneous rock into Mt. Whiteface, which is also a syenite. 



* The map accompanying the Hitchcock report shows Mount Tecumseh 

 as composed of granite, but this is wrong, for certainly the greater part of 

 it is mica schist. 



