ON COARSE GABBROID DIABASE IN WESTFIELD, 

 MASSACHUSETTS^ 



EARL V. SHANNON 



United States National Museum 



In the western part of the town of Westfield, Massachusetts, a 

 number of quarries have been opened in the Holyoke diabase sheet. 

 These quarries are the same which have furnished the beautiful 

 datoUte specimens which are to be found in all collections. The 

 diabase or trap in which these quarries are located is the middle 

 or main extrusive trap sheet of the Connecticut Valley Triassic 

 area. Here the main portion of the sheet is composed of a gray 

 holocrystalline diabase of fine to medium grain not unlike the 

 intrusive traps which form East and West Rocks near New Haven. 

 In the northernmost of the four quarries there occur, included in 

 the normal rock, irregular areas of a gabbroid rock very unusual in 

 appearance for an extrusive lava. While studying the occurrence 

 of datolite and other secondary minerals in these quarries, the 

 writer's attention was attracted to these very coarse-grained phases 

 of the sheet and they were examined in some detail. At first it 

 was believed that they represented a later intrusion which had 

 chanced to penetrate the previously existing flow, but a study of 

 the relations of the coarse material to the surrounding diabase of 

 normal grain led to the abandonment of this view. The contact 

 with the surrounding rock is sharp, the transition from coarse to 

 fine material being accomplished in a distance of an inch or less. 

 The coarse-grained gabbro forms irregular rounded areas often 

 many yards in diameter in the finer-grained diabase. In the hand 

 specimen this coarse rock shows broad blades of bronzy greenish- 

 black pyroxene which reach an inch or more in length, imbedded 

 in a coarse granular aggregate of pyroxene and greenish feldspar. 

 Under the microscope the rock is seen to be composed of large 



'Published by permission of the Secretary of the Smithsonian Institution. 



579 



