MINERALOGY AND PETROLOGY. 969 



" Primary '* and " secondary " minerals.] 



foregoing mentioned minerals. The basic magma might acquire acid elements, both 

 before and during the stage of cooling, and the resulting minerals might date all 

 along from the moment of initial solidification to the moment of complete cooling. 

 Quartz, in particular, would be very facile under these forces, and it would be utterly 

 impossible to say at what stage in the history of the consolidation and generation of 

 the rock it took its place in the mass. Again, if the basic magma surrounded and 

 incorporated fragments of acid rock, to what extent would the resulting minerals 

 justly be called original, and to what extent secondary? Some of the included acid 

 material would so penetrate some of the pre-existing minerals as to change them to 

 others. Some basic plagioclases appear thus to have been converted into red feld- 

 spars which pass for oligoclase and orthoclase. Augite, under such circumstances, 

 is converted to hornblende and to chlorite. Therefore, as to date and method of 

 genesis there is no criterion to guide in the determination whether such minerals 

 should be called primary or secondary. They are essential parts of a recognized 

 rock mass and they are chemically more stable and enduring than some of the other 

 minerals of the same mass which are recognized as original. The rock in which 

 they exist sometimes is of large amount, and it may have been given specific name, 

 a circumstance which requires that they be regarded as original, whatever their 

 origin or date. 



Whatever the uncertainty and confusion that may have attended the generation 

 of these minerals, when considered from the view points of original and secondary, 

 there seems to be one common element in which they share, and which might furnish 

 them a classificatory designation, viz.: they are the product of mineralizing agents 

 during the cooling stage of the rock. 



In order to make plain the ideas of the writer, and net to disturb the prevalent 

 practice of petrographers in the use of the term secondary, the following sketch of 

 the successive dates of the minerals of the igneous rocks of the Keweenawan may 

 be presented: 



1. The primary minerals in these rocks are those of the first consolidation, 

 usually porphyritic. 



2. Those of the second consolidation, composing the mass of the rock not 

 altered by gases incident to the cooling epoch. 



3. Minerals produced after consolidation, during the process of cooling.* 



4. The latest minerals formed, i. e., those resulting from atmospheric agents 

 after cooling, i. e., the minerals resulting from atmospheric decay. 



*Teall speaks briefly of the fact that the cooling period of an igneous rock must necessarily be one of long duration, but he 

 does not make it, so far as noticed, the birth-date of any of its minerals. 



M. M. Fouque and Miehel Levy, though they make provision for two secondary stages of mineral generation (Mineralogie 

 inii-rou'raphic, 1879, pp. 151, 152), confine their exposition to primary and secondary stages of cunsitlitldtiini, without even giving a 

 definition of the secondary processes, which are simply liste.l : 



"it. Actions secondaires immediates. 



"4. Actions secondaires mediates." 



