152 MINERALOGY AND LITHOLOGY. 



formed Hitchcock's flume in the Notch. In this rock the feldspar is 

 much altered; the augite is entirely decomposed into chlorite and cal- 

 cite, and the magnetite is in crystals. At Rye, a diabase occurs that 

 combines almost all the peculiarities of decomposition that have been 

 mentioned. It contains labradorite, augite, magnetite, and apatite as 

 original constituents, and chlorite, hornblende, biotite, epidote, and calcite 

 as secondary. It also contains pyrite, which is mixed with the magnetite. 

 And thus we might go on, for every rock presents some variations pe- 

 culiar to itself. The variations are, however, those of different propor- 

 tion in the original constituents, and in the relative amount of decompo- 

 sition products of the various kinds mentioned, and description becomes, 

 therefore, the endless repetition of the same idea. 



The next most prominent variety of massive diabase which occurs in 

 New Hampshire may be called mica diabase. The difference between 

 this and the last variety was not originally great, but the mode of altera- 

 tion has widened the gap between them. In this rock, the decomposing 

 agencies have produced no chlorite of consequence, and hence the rocks 

 are not green, but of a light gray color. In appearance they are as fresh 

 as if crystallized yesterday. On being moistened with dilute acid they 

 do not effervesce, and in looks and behavior quite surprise one who is 

 looking for old weather-beaten rocks. But the microscope indicates as 

 much alteration here as in the first case, for, on applying polarized light, 

 what was originally augite is seen to be no longer a homogeneous min- 

 eral, but an aggregate of minute crystals that resemble calcite. On treat- 

 ing the rock with dilute acid it does not effervesce ; but if the acid is 

 heated it effervesces long and powerfully, and in the solution lime, iron, 

 and magnesia are abundantly found. This indicates the formation of 

 dolomitic and ferruginous carbonates by decomposition, and makes it 

 plain why the rock looks so fresh. The rock is quite feldspathic, and the 

 feldspar is undecomposed, and its bands of color arc clear and distinct. 

 This fact, united to the circumstance that the augite is converted into 

 carbonates, accounts for the light color and fresh appearance of the rock. 

 The appearance of a section of a specimen from Bemis brook, in ordinary 

 light, is represented in Fig. 4 on PI. 6. Biotite is a characteristic mineral 

 in this variety of diabase, and is very consioicuous in thin sections. It 

 exists in little scales, which, when lying in the plane of the section, are 



