13-4 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



sible chance in a section that no crystals shoiihl be intersected in a 

 l)lane possessing the apiiroximate maximum angle between the traces 

 of the twinning plane and elasticity plane, and there is also a possibility 

 ^liat one might examine many sections with approximately equal angles 

 of extinction to the right and the left of the twinning plane, as the 

 method requires, without meeting sucn as possess an angle characteristic 

 of the species, even were such present; hence the correct determination 

 of the species by this method must remain to a certain degree a matter 

 of chance. But when the feldspathic element is complex, a determina- 

 tion based upon this method would lead one to determine the whole of 

 the feldspar as belonging to the species with the maximum angle of ex- 

 tinction. This method has been applied to a very considerable extent, 

 but the considerations here advanced show that the method, although 

 entirely correct in principle, and certainly of some value in lithological 

 research, is not adapted to the final determination of the exact nature 

 of the feldspathic constituent, and is likely to lead to erroneous conclu- 

 sions. 



All are familiar with the grand diversity of the secondary products 

 that occur in these rocks, and which have enriched the mineral cabinets 

 of the whole world. The contact, modifications, and structural features 

 are none the less interesting ; but it is not the intention to consider 

 these questions here. It will be well to record the observation that they 

 are sometimes so modified by quick cooling upon the exterior walls of 

 the dikes in contact with the sandstones that they become crypto- 

 crj'stalline, and contain long acicular feldspar crystals in a glassy ground 

 mass, and in other microstructures resemble augite andesites. The oc- 

 currence of glass has not been heretofore noticed in these rocks. Ex- 

 cepting such local modifications the rocks are always like the ordinary 

 old diabases, and even in microscopic features monotonously alike 

 wherever fresh stones occur. 



These rocks so characteristic of our Triassic are additionally interest- 

 ing on account of the comparative absence of eruptive material intruded 

 in the similar Triassic sandstones of Europe. 



National Museum, Washington, D. 6'., April 5, 1881. 



OIVTHE ]>ET£:R9IIIVATI0]V of FEL.08PAKIIV THI]V !!iECTIOIVS OF ROCKS. 



By GEORGE W. HA\» E§, PI». I>. 



In my recent article upon the composition of the diabases * which in- 

 tersect the INIesozoic red sandstones upon the Atlantic border, I sug- 

 gested that the complexity of the feldspathic element in basic rocks is 

 probably much greater than is commonly supposed, and that this com- 

 I)lexity is liable to cause a serious error if the method of determination 



* This volume, page 129. 



