446 Pirsson and Washington — Geology of Red Hill, IV. H. 



merits and larger irregular crystals of the nephelite-syenite as 

 well as the actual pieces of the rock previously alluded to. 



Under the microscope this dark cement is found to consist 

 of oli vine-brown hornblende and alkalic feldspar with occa- 

 sional tablets of biotite, rare prismoids of a pale gray-green 

 diopside or scattered crystals of sphene. It is interesting to 

 note that it contains little or no iron ore, the iron being all in 

 the hornblende. The fabric tends to be equi-granular, the 

 grains equiform and roughly rounded. The enclosed feld- 

 spars of the syenite have extremely irregular, ragged boundaries, 

 otherwise they show no change. A significant circumstance 

 is that between some of the included rock fragments, the min- 

 erals of the cementing rock are arranged in such a manner as 

 to indicate flowage structure, from which they are more or less 

 orientated. 



Summary and CoNCLUsroxs. 



The study of Red Hill shows it to be a mass of nephelite- 

 syenite intruded into granite-gneiss. Its intrusion was fol- 

 lowed by smaller upthrusts of magmas more salic and more 

 femic, which appeared in fissures in the syenite and in the 

 zone of cracked and shattered gneisses about it, forming a 

 series of complementary dikes. Since the dikes were not 

 found cutting one another their relative sequence cannot be 

 given. Their general characters are such as to point clearly to 

 their magmatic relationship to the main mass of syenite which 

 they accompany. This is shown by their mineralogical and 

 chemical nature and is the same relation which has been 

 repeatedly observed in other regions. Thus, while the study 

 of the area affords the weight of additional evidence to the 

 genetic connection which in recent years has been seen to 

 exist in complexes of igneous rocks, it offers nothing new or 

 especially striking in this regard. One feature however 

 deserves further attention. It has been shown that the outer 

 portion of the mass is decidedly more siliceous than the inner- 

 one, the latter being a nephelite-syenite with Si0 2 , 58*3 per 

 cent, while the outer one is slightly quartzose-syenite in which 

 the Si0 2 cannot be less than 66 per cent. 



It has also been shown that there is probably some vertical 

 difference of the same kind, the top being more siliceous than 

 the inner portion. While this does not appear to be the case 

 ordinarily seen in intruded masses, which usually have the 

 outer zone more basic or femic than the interior, yet a num- 

 ber of such instances are known, one of the latest which has 

 been described being in the massif of the neighboring Belknap 

 Mountains as shown by Dr. Washington and the writer.* 

 * This Journal, vol. xxii, p. 509, 1906. 



