LITHOLOGY. 175 



how commonly all these things are to be seen ; and with this short expla- 

 nation, intended to show unity where much diversity is apparent, we will 

 proceed to the description of the more marked varieties of these rocks. 



Felsites, porphyritic felsites, granites, and sienites are the rocks that 

 are considered in this general division. 



Felsite. 



Felsite is a very fine-grained, compact rock, the composition of which 

 is not at all evident to the eye on account of the minuteness of the crys- 

 tals of its constituent minerals. In New Hampshire this rock is usually 

 found in small dykes, and it usually has a white or gray color. It was 

 long ago noticed that fragments were fusible before the blow-pipe, from 

 which circumstance the nature of these dykes was suspected before it 

 was known. In thin sections under the microscope it is seen to consist 

 of an intimate mixture of quartz and orthoclase feldspar. It is the same 

 substance as that which forms the ground mass of the porphyries, and 

 hence its study forms a fitting introduction to that of those rocks. 



Felsitic substances are divided into two classes, according to certain 

 microscopic properties which were first pointed out by Zirkel. The first 

 class embraces those compact mixtures of quartz and feldspar, which, 

 even under the microscope in thin sections, cannot be resolved into its 

 constituent minerals, and would be called noncrystalline, except that be- 

 tween crossed Nicols it does not become entirely dark, but shows faintly 

 the optical properties of an aggregate. Glassy matters are, moreover, 

 often present in such rocks. These rocks are called micro-felsitic. The 

 rocks of the other class are to the eye homogeneous; but when thin sec- 

 tions are magnified they are seen to consist of well individualized though 

 minute grains of the constituent minerals. These rocks are called macro- 

 felsitic; and to this class all our felsites belong, for, although some of 

 them are extremely fine in texture, they can all be resolved into granular 

 mixtures when sufficiently magnified. The rocks that occur about Mt. 

 Washington may be described as typical. 



In going up the Mt. Washington river towards the gabbro rocks, sev- 

 eral dykes of felsite are met with. They are white in color, and when 

 freshly broken usually show some bright green spots, which at one time 

 were cavities, but which are now filled by an aggregate of quartz and 



