TYPES OF FOREIGN INCLUSIONS 95 



vance of the mass, but this would seem to involve a sufficient thickness 

 of the crust to prevent the blending of this mud with the still liquid 

 trap beneath. 



Unlike the first case, this process has in the cases previously described 

 produced no glass and no metamorphic effect. We may call this the 

 Titans Pier type. The mud has been mingled with the lava superficially 

 in great quantity at the lowest possible temperature and pressure, and 

 the heat has acted for the shortest time. This explains the difference in 

 the results between this and the Meriden type. 



HOLYOKB RESERVOIR TYPE: BLENDING OF MUD AND LAVA IN THE CENTRAL 



PORTION OF THE SHEET 



General statement. — In the case now to be described the mud which in 

 the immediate vicinity is spread over the surface of the trap seems to 

 have been also drawn down more than a hundred feet into the interior 

 of the mass, and thus, while subjected to increased pressure, time, and 

 heat, to have produced physical and chemical changes as intense but 

 very different from those of the Meriden type. 



Description of the area. — The map (see plate 24 and section) shows a 

 portion of the main trap sheet h\ miles long. At the south end, in the 

 extreme southwest corner of Holyoke, at Dibbles Crossing, is shown the 

 superficial area of the trap, which is filled with inclosures of drab lime- 

 stone and shale, after the Titans Pier type. South of the Dibble house 

 the railroad cut shows the structure perfectly. In the ridge just south- 

 east and also in the brook cutting beneath the next house south * the 

 sandstones can be seen resting on the diabase-limestone emulsion and 

 the exposures are entirely satisfactory. A half mile northwest, out on 

 the back of the deeply eroded trap ridge, is the area in question of the 

 Holyoke reservoir type. It is about 1 mile long and 40 rods wide in its 

 southern part. Topographically it is a quite deep depression in the trap, 

 more so than the contour lines indicate, and was natural^ chosen for a 

 reservoir. North of the road the depression is continued across swampy 

 and covered ground a mile farther north, where it becomes a deep notch 

 in the trap in which the glass-bearing rocks of the reservoir again occur 

 and have been blasted into for the electric road. 



The section shows that the area is about 150 feet below the upper 

 surface of the trap sheet, though the possibility of faulting is not 

 excluded. 



The normal diabase. — The trap in all this portion of the great sheet is 

 the typical dark and fine grained Holyoke diabase. It is exceptionally 

 fresh and compact, but is everywhere cut within the given area by 

 schlieren or segregations of peculiar coarse-grained diabase types. 



♦See figure 1 in "Diabase pitchstone," etc. Loc. cit., p. 62. 



