LITHOLOGY. I85 



resting quietly in their original beds. If the eruptions reached the sur- 

 face of the earth the pressures would be relieved ; and as the cavities 

 point as before to very heavy pressures, we are led to suppose that such 

 disturbances are the result of movements which took place in the earth's 

 depths rather than upon its surface. The circumstance of the elevation 

 of our mountains is sufficient to make plain the possibilities of such sub- 

 terranean movements. This is an interesting spot, for such a mixture 

 of stratified and unstratified material is not often seen. 



Quite a variety of quartz porphyries are found in this region. Some 

 are colored red by the oxide of iron that seems to have separated from 

 the decomposing minerals ; in some, but few crystals are porphyritically 

 developed, and some others are nearly granitic in their texture. On the 

 lower half of the mountain a gray porphyry is prominent, which possesses 

 a very fine, compact, rather earthy-appearing ground mass, in which not 

 only feldspar crystals are developed, but also numerous dihexahedrons of 

 quartz. Such porphyries as this were once called clay-stone porphyries, 

 on account of the appearance of the ground mass. Another specimen 

 of this rock has a basis still more earthy in appearance, which results 

 from the abundant presence in it of chlorite, biotite, and iron oxide, 

 which are only microscopically identified. Moat mountain, and some 

 other mountains surrounding Pequawket, are largely composed of por- 

 phyries. 



A porphyry just like this, save that in it very much larger crystals of 

 orthoclase and quartz are developed, occurs in the Notch in the White 

 Mountains. This porphyry is considered by Prof. Dana to form a bed, 

 the position of which is conformable with the strata of slates and schists, 

 and thus it presents a case of a porphyry which was not disturbed from 

 the place where its metamorphism took place. 



A most diversified collection of porphyries can be obtained at Water- 

 ville. They are of various colors, owing to combinations of differently 

 colored ground mass and crystals, but the prevailing color is red. This 

 color is seen in thin sections to be due to oxide of iron, which is abun- 

 dant in the ground mass, and is often spread through the minute fissures 

 in the orthoclase crystals. Sometimes a black grain of iron oxide in a 

 strong light will appear blood-red, and it is thus recognized as hematite. 

 These porphyries also vary widely in composition. In some, quartz is 

 VOL. IV. 24 



