PETROLOGY AT GOOSE CREEK — SHANNON. 



11 



a (100). Cleavage parallel to m (110) is well developed. The py- 

 roxene from all parts of the quarry is of uniform composition, as 

 shown by the constancy of optical properties. That from the coarse 

 (2 mm. grained) rock which has been mentioned as being the nor- 

 mal rock at the north end of the quarry was separated for analysis 

 by the use of heavy solutions and an electromagnet, yielding a prod- 

 uct which was homogeneous pyroxene although not entirely free 

 from fine dustlike inclusions. Upon analysis this gave the results 

 and ratios of columns I and II of the following table. Analyses of 

 pyroxenes separated from other Triassic diabases are quoted, for 

 comparison, in the other columns of the table. 



Analyses of pyroxenes separated from Triassic traps. 



Constituent. 



SiOj 



Ti02 



AI2O,, 



Fe^O'a ---. 



FeO 



MnO 



CaO 



MgO 



(Na, K)20. 

 H,0 



50. 26 



.80 



2. 10 



None. 



18.20 

 . 35 



15. 56 



13. 30 



0. 8381 

 .010 0. 869 

 .O21J 



47. 72 



48. 54 



50. 71 



.2531 



.006 07, 

 .279 -^^^ 

 . 333J 



3.44 



5. 93 



18. 34 



12. 89 

 1. 23 



5. 50 



2. 77 

 21. 25 



10." 97" 



7. 67 



3. 10 



.82 



3. 55 



15. 30 



.81 



13. 35 



13. 63 



1.48 



1. 17 



Total 



100. 57 



100. 95 



100. 62 



100. GO 



1. Pyroxene separated from diabase of Goose Creek, Va. E. V. Shannon, 

 analyst. 



2. Ratios of analysis 1. 



3. Pyroxene from diabase of Rocky Hill, N. J. A. H. Phillips, analyst. 



4. Pyroxene, Rocky Hill, N. J. A. H. Phillips, analyst. 



5. Pyroxene, from diabase of West Rock, New Haven, Conn. G. W. Hawes, 

 analyst. Last three analyses quoted from J. Volney Lewis, Ann. Rept. State 

 Geologist of New .Jersey for 1907, p. 117. 



Wliile the normal fresh pyroxene of the Goose Creek diabase is 

 characterized simply by the m (1 10) cleavage and twinning on a (100) , 

 most sections show a distinct very fine lamination parallel to the 

 base c (001). This seems to be due to strains or pressure and in its 

 incipient stages is only a faint parting or cleavage in the transparent 

 crystals. With further development this becomes a strong parting 

 with very minute polysynthetic twinning on this plane. These part- 

 ing and twinning planes are particularly favorable to alteration and 

 are frequently marked by innumerable opaque, minute, dust-like 

 grains, probably of magnetite or ilmenite. With more alteration, films 

 of chlorite are usually developed along these partings. The optical 

 properties of the pyroxene are uniform with the indices of refraction 



