THE HOLYOKE SHEET. 4(33 



discussion is one of the freshest-looking in the viille}^, and yet it is some- 

 times impossible to find in a slide a single feldsjjar on which one can 

 observe the extinction, so decomposed are they, and the series of which 

 the extremes are given above are taken from the whole length of the 

 valley. 



The augite is strictly subsequent to the lath-shaped feldspars and 

 presents little that is specially noticeable, though oftentimes it is less 

 decomposed than the feldspars. It differs thus in the large sheets from 

 the diabase of the tuff above and of the newer dikes, where the augite is 

 often porphyritic and contemporaneous with the earlier feldspars. 



I have in many places noted olivine with a query; but on re^-iewiuo- 

 the whole series of slides I have not been able to find either the unchano-ed 

 mineral or any serpentine or hematite patches which would seem to have 

 been derived from it at the locality under consideration or in either of the 

 large trap sheets. In the dikes in the gneiss and in the newer dikes in the 

 sandstones it occurs, and it may be wanting in the large beds only because 

 of their advanced state of decomposition. 



Magnetite is uniformly distributed, always rather but never very 

 abundant, generally quite well crystallized. The delicate featherwork of 

 beaded octahedra is especially abundant at the base of the great bed at the 

 contact on sandstone just north of Titans Piazza. 



Apatite, never alnmdant, is rarely to be detected except piercing 

 magnetite. 



There is no trace of groundmass discernible between the constituents; 

 rounded or pear-shaped blebs of glass appear in the older feldspars. 

 Cavities filled with diabantite, rust, calcite, and zeolites are not wanting, 

 even in the wholly compact rock we have chosen for discussion, but they 

 are very minute. Sections from the upper surface of the dike where it 

 is cut by Dry Brook in the northwest of South Hadley exhibit very 

 beautiful amygdules, showing from without inward diabantite, calcite, and 

 radiated uatrolite. 



CHEMICAL COMPOSITION. 



In 1838 President Hitchcock analyzed the much decomposed and 

 amygdaloidal trap from the east end of Mount Holyoke with the result 

 shown in column 1.^ In 1875 Dr. Gr. W. Hawes published analyses of 



' Economic Geology, p. 135. 



