PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 99 



Qabbro.] 



It embraces the labradorites ophitically but never the orthoclases. On the contrary, 

 the orthoclase surrounds it, in a manner somewhat similar to that in which it sur- 

 rounds the labradorites. There is, however, an important difference. When the 

 pyroxenic element is surrounded by the orthoclastic the former is uniformly and 

 deeply changed, and converted into chlorite, magnetite, quartz, while the labradorites 

 have usually maintained their forms and chemical integrity. The formation of the 

 orthoclase and the quartz seems to have been consequent, or at least coincident, with 

 the demolition of some of the pyroxene grains. In these sections there is no evidence 

 of the mechanical intermixture of any of the foreign (acidic) elements from the 

 clastic strata, but only of such chemical transformations as the near proximity of 

 such minerals under pressure, heat and moisture may have promoted. This transfor- 

 mation took place, of course, during the cooling period. It may not be inferred that 

 the change in the pyroxenes was due to the action of these acidic transfusions, because 

 similar changes, except the introduction of quartz, took place in the same mineral 

 where it was not subjected to such influence. It is only allowable to infer that the 

 cooling period was the date of activity of both agents of change, and that they com- 

 bined in No. 5, and added quartz and orthoclase to the usual products. 



Magnetite is of two dates, as in No. 1C. By far the larger portion, however, is 

 of secondary (cooling stage) date, and has taken the shape of the original pyroxenes 

 or of a glassy residue. See figure 4. 



FIG. 4. SECONDARY MAGNETITE FROM NO. 5. 

 The included grains are of clear feldspar. 



Figure 4 is a reproduction of a drawing of a part of a mass of magnetite in No. 

 5, made by the use of the camera lucida. It shows the form which the magnetite 

 sometimes presents. A fracture line in the section separates this from another 

 portion of the same grain of magnetite, the latter showing similar outlines and 

 inclusions, though not so large as this. The included parts are bright feldspar, which 

 also surrounds the whole mass, there being no augite in contact with it unless it be 

 represented by chlorite scales, which are seen about the larger end. The smooth, 

 rounded outline of the border, and the general absence of pyroxene and its residue, 

 suggest that this may have taken the place of some of the uncrystallized magma, yet 

 its ophitic relation to the feldspar is more like that of pyroxene. 



