MINERAL COMPOSITION OF DIORITES. HI 



Table V presents those varieties of the stock rocks in which the amount 

 of the ferromagnesian silicates about equals that of the feldspar and quartz 

 combined. There is no distinction made as to whether the crystals occur 

 as phenocrysts or not. They are arranged in a series according to their 

 degree of crystallization, the finest grained being at the top, the A^alue of 

 the degrees of crystallization having been already explained (p. 99). The 

 silica percentage is given in all cases where it has been determined. In the 

 table an attempt is made to express the relative amounts of the quartz, of 

 the hornblende and pyroxene, and of the biotite and- hornblende and 

 pyroxene. The relative amount of feldspar is not expressed. In a general 

 way it varies inversely as the amount of quartz for this subgroup. The 

 columns under the different divisions of the table express certain relations 

 of the minerals approximately. Under the divisions of quartz, the terms 

 "little," "moderate," "considerable," "much" are used only as comparative 

 terms applicable to this group of rocks throughout its three subdivisions, 

 II«, lib, lie, and have no reference to the relative amount of quartz which 

 might be found in another suite of rocks. Consequently, what would be 

 considered "much" quartz in these rocks might be only a moderate amount 

 for another series. 



Under the division which shows the relative amounts of pyroxene and 

 hornblende in each rock, the first column, u p" indicates that there is 

 pyroxene and no hornblende; the next column, that the pyroxene is in 

 excess of the hornblende; the third, that they are equal; and so on. The 

 relative amounts of pyroxene or of hornblende in any two varieties of the 

 rock is not indicated directly. It can be ascertained roughly by consider- 

 ing that in this subgroup the sum of the pyroxene, hornblende, and biotite 

 is nearly constant. 



In the next division of the table the amount of the biotite is compared 

 with that of the pyroxene and hornblende combined, in the manner already 

 explained for the previous division. 



The first fact brought out by a study of this table is the variability of 

 the quartz percentage, which does not appear to hold a very definite relation 

 to the silica percentage, as in the case of Nos. 281 and 282. But it is 

 observed in studying the thin sections that the quartz is not so noticeable in 

 the fine-grained varieties as in the coarse-grained ones, and may therefore 

 be either overlooked or possibly not so strongly developed. Thus the 



