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 mca 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 are 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 Pl. 6. Biotite is a characteristic mineral 
in this variety of diabase, and is very conspicuous in thin sections. It 
exists in little scales, which, when lying in the plane of the section, are 
